Consensus on the best mid-fi Hifiman to get these days?

[enricoclaudio]

I got the Sennheiser HD660S and HIFIMAN Sundara with the intention of returning one. Guess what? I'm keeping both. Can't let any of the two go. People keeps comparing these two headphones with no reason, at least to me, as they are totally different cans (in a good way). Both are GREAT headphones and worthy of owning both. I'm driving them with a Topping A30Pro and Matrix Audio mini-i Pro 3.

 

[Robbo99999]

Consensus on the best mid-fi Hifiman to get these days? I don't want to go over $200, and I prefer a neutral sound. Amp will be JDS Labs Atom.

I want something that competes with the Senn HD-600.

Thank y'all!

So, if I've understood your situation, you can exchange your pair of phones for simething else at Hifiman and 130$ will be knocked off the price of that something new?

And the 200$ limit you have is the total, which would mean the most you would do is another 70-ish$ above the 130$?

If that is the case, and looking at what measurements are available, the 400SE seems fine.

That being said, if your pair are not defective, you may not be getting an upgrade.

If you're willing to add more money nothing under the Sundara makes sense.

You could always sell what you have (I've read your concerns, it really is not a lot of money, I've sold much more expensive stuff over ebay). If it sells for, say 100$ then you can add another 100$ and have more flexibility.

As people have mentioned, the 6xx becomes an option but personally I'd recommend the HD560S. I have them and the Sundaras, they have better bass extension and measure really neutral, compete very well with the Sundara at 200$.

Someone already mentioned, don't fall into the rabbit hole. You've used some of the language: "elite reviewer" for example, which hails the start of audiophoolishness - but hopefully coming to this forum will nip that in the bud. Atom stack + the 560S is affordable neutral clean sound and you can always EQ to change the sound if you prefer something different since the foundation is clean.

I agree about the HD560s, it's my favourite headphone, for me it's a better headphone than the Hifiman HE4XX. HD560s has better soundstage than both HE4XX & HD600/650/6XX, and HD560s has cleaner bass than the HD600/650/6XX and nearly as good in the bass as the HE4XX. Talking when all headphones EQ'd to the Harman Curve. HD560s also has better "detail" than HE4XX. I own the HD600 / HE4XX & HD560s, as well as a couple of others. @shaney777 I'd recommend the HD560s rather than the Hifimans in your budget and indeed over the HD600/650/6XX that you were lusting after but couldn't afford.

 

[shaney777]

It’s possible to over complicate these things, and most reviewers are an obstacle to simplifying things. My advice is write down a very short list of things that you want/don’t want in a headphone. Sounds like ‘neutral’ is the main thing. Think about your use case when you do this e.g how noisy is your listening situation-would closed or open be better. Once you have a short list meaningful criteria, which can include cost, judge against that and that only. Forget subjective reviews not correlated to measurements-they don’t mean anything.

My feeling is that what you are looking for is obtainable for $200 or less. Only when you have chosen a headphone do you think about your amp. You can’t really beat the atom at its price.

Bear in mind that no matter how good your set up, a legion of subjective reviews are conspiring with your cognitive biases to make you think that the thing you decided on rational grounds was good 5 minutes ago isn’t. Don’t listen to them, they will stop you from enjoying a perfectly good setup.

Thank you for the advice! I agree I get caught up in bad reviews for any item I am interested in. You can't go on Amazon and find anything that hasn't been torn apart by people. It makes buying stuff extremely difficult because usually they are talking about some really serious flaw. Frustrating!

 

[shaney777]

So, if I've understood your situation, you can exchange your pair of phones for simething else at Hifiman and 130$ will be knocked off the price of that something new?

And the 200$ limit you have is the total, which would mean the most you would do is another 70-ish$ above the 130$?

If that is the case, and looking at what measurements are available, the 400SE seems fine.

That being said, if your pair are not defective, you may not be getting an upgrade.

If you're willing to add more money nothing under the Sundara makes sense.

You could always sell what you have (I've read your concerns, it really is not a lot of money, I've sold much more expensive stuff over ebay). If it sells for, say 100$ then you can add another 100$ and have more flexibility.

As people have mentioned, the 6xx becomes an option but personally I'd recommend the HD560S. I have them and the Sundaras, they have better bass extension and measure really neutral, compete very well with the Sundara at 200$.

Someone already mentioned, don't fall into the rabbit hole. You've used some of the language: "elite reviewer" for example, which hails the start of audiophoolishness - but hopefully coming to this forum will nip that in the bud. Atom stack + the 560S is affordable neutral clean sound and you can always EQ to change the sound if you prefer something different since the foundation is clean.

Thank you for the advice! Yes, you understand everything correctly. As of now, I am tempted to have Hifiman send me a new replacement, sell it somewhere, then pick up an HD-600. This has always been my dream headphone, so it's hard to choose anything else. All that's stopping me is the amp... Basically no one is talking about an Atom and HD-600 combo, which worries me. People over at Head-fi have always chanted OTL tube amp for them, and I really can't afford that.

 

[shaney777]

This should be a no-brainer :

• Hifiman Sundara (2020 revision) : $350 (since you need to stay within Hifiman, there's their best headphone in your price range)
• Topping E30 DAC : $120 (E50 if you want to go balanced)
• Toppinc L30 amp : $130 (L50 if you want to go balanced)

Total : only $500, and you don't get any better than that, objectively. Measurements prove it.
Last but not least, EQ the headphones and see the difference. It's much more important than 'tubes' or anything supposedly 'magical'.

There's no point in buying an overpriced Bottlehead Crack, whatever "people" may say. It's nonsense. For the record I have a $2000 tube speaker amp with an expensive Siltech power cable. Because I too was fooled once.

I really appreciate the breakdown. How do you feel about the HD-600 vs. Sundara in terms of sound quality? These are going to be the same price, and one is my dream headphone.

 

[shaney777]

I agree about the HD560s, it's my favourite headphone, for me it's a better headphone than the Hifiman HE4XX. HD560s has better soundstage than both HE4XX & HD600/650/6XX, and HD560s has cleaner bass than the HD600/650/6XX and nearly as good in the bass as the HE4XX. Talking when all headphones EQ'd to the Harman Curve. HD560s also has better "detail" than HE4XX. I own the HD600 / HE4XX & HD560s, as well as a couple of others. @shaney777 I'd recommend the HD560s rather than the Hifimans in your budget and indeed over the HD600/650/6XX that you were lusting after but couldn't afford.

Thank you! I will look into them. I know zero about this headphone! I assume the Atom would power them sufficiently?

 

[Skinner001]

@shaney777
I wouldn't get caught up by what folks at Head-Fi say - there's plenty of audiofoolery going on there. Solid states are superior to tube amps at, well, amplification (taking the signal, not messing with it and amplifying it) - simple as that. People that don't bother spending money and like collecting gear might like to have some tube gear - others might enjoy the particular flavour of distortion from tubes and then tube roll to get just the right kind of sound for them - but it's just that, distortion - go for clean sound, especially when just starting.

JDS should do fine, but if you want a bit more power at a similar price then look at Schiit Magni 3 and Heresy + their Modi dac. The newest versions measure well (even though Schiit does some bad measuring shit and can be jackasses).

In my experience, the Sundara is more open (simply due to build) and leaks more sound than Sennheiser, but also has a bit more pronounced treble and the HD600 will be smoother in that regard. I won't say the Sundaras are piercing in the treble, but if you're sensitive to those sorts of things, you may come upon some peaks form time to time. Obviously, if you're up for EQing then you can tune to your preference.

Ideally, you could get both, try them out for a week and return the ones you like less.

In my experience a good way of going about things is:
1. Look at measurements and don't believe golden eared audiofools with fluffy language and ill-defined, inconsistent terminology
2. Using measurements and your budget - narrow down the selection
3. If possible - demo the selection - there's a lot of stuff that matters - comfort for you personally, the build quality, even looks - for example, the AKG371 are a good measuring, affordable closed back - but I just can't use them because for me the fit is not good and I dislike the build intensely
4. Give yourself some time - I found that over a sufficient time span I notice I'm grabbing one piece of gear more often than another and if something is not being used it's time to sell/return - now, if you don't have the luxury of time - you'll be fine since you started out looking at good measurements
5. Dial in your taste - you may prefer more bass, more treble, or less of both - if you start off with good measuring gear you can dial in your taste via EQ - this is the point that most subjectivists can't seem to get - they want "flavour" but they seem to want it in their gear - which is all fine and dandy if you have money to burn and really like spending on gear - just not the way most people should do it.

Obviously, most of the people here like the hardware itself - not just enjoying music - otherwise we wouldn't be spending so much time, effort and money in this hobby - but my advice is for that balancing act to favour listening to music (or enjoying other sound - e.g. movies, games) as much as possible - you'll find you can get good, clean gear for not a lot of money and enjoy your music just fine. Once I started making this thought prominent in my mind - I could more easily sell stuff I wasn't really using (e.g. Focal Clears, and all but one AMP and DAC), and I feel like I don't have to chase the next great thing. Some of us learned by making mistakes and starting off as audiofools - probably best to learn from our mistakes rather than making your own

 

[Robbo99999]

Thank you! I will look into them. I know zero about this headphone! I assume the Atom would power them sufficiently?

Yes, definitely, I have the Atom Amp myself.....in fact you don't even need a "proper" headphone amp to power the HD560s, they're very easy to drive.

 

[Robbo99999]

Thank you for the advice! Yes, you understand everything correctly. As of now, I am tempted to have Hifiman send me a new replacement, sell it somewhere, then pick up an HD-600. This has always been my dream headphone, so it's hard to choose anything else. All that's stopping me is the amp... Basically no one is talking about an Atom and HD-600 combo, which worries me. People over at Head-fi have always chanted OTL tube amp for them, and I really can't afford that.

Doesn't matter much which headphone amp you have as long as it's been measured by Amir on this site and has measured well. I have the Atom Amp and HD600 and of course it drives them fine. I think you're wasting your time/money choosing the HD600 though, with the exception being if you know you're not fussed about soundstage & size of soundstage - they've got a small soundstage. HD600 are one of the best headphones for everything else though, apart from the bass which is a bit woolly & ill-defined when boosted. (HD560s as per my last post is the better headphone all-in-all).

 

[Eldgrim]

Basically no one is talking about an Atom and HD-600 combo, which worries me.

From what I remember a lot of people tested it with the HD6XX which has the same power reqs as the HD600, and it worked well with no issues. Specs also indicate that it shouldn't be an issue.

Thank you! I will look into them. I know zero about this headphone! I assume the Atom would power them sufficiently?

More than sufficient for HD560s. It is quite easy to drive.

 

[Eldgrim]

Here are some measurements to show the difference in frequency response between the HD600 and HD560s. Almost identical except in the bass and around 4-6k. The bass under 100hz is really lacking on the HD600. If the HD560s has a larger soundstage then I would go with that as the HD600 series basically don't have any.

 

[shaney777]

@shaney777
I wouldn't get caught up by what folks at Head-Fi say - there's plenty of audiofoolery going on there. Solid states are superior to tube amps at, well, amplification (taking the signal, not messing with it and amplifying it) - simple as that. People that don't bother spending money and like collecting gear might like to have some tube gear - others might enjoy the particular flavour of distortion from tubes and then tube roll to get just the right kind of sound for them - but it's just that, distortion - go for clean sound, especially when just starting.

JDS should do fine, but if you want a bit more power at a similar price then look at Schiit Magni 3 and Heresy + their Modi dac. The newest versions measure well (even though Schiit does some bad measuring shit and can be jackasses).

In my experience, the Sundara is more open (simply due to build) and leaks more sound than Sennheiser, but also has a bit more pronounced treble and the HD600 will be smoother in that regard. I won't say the Sundaras are piercing in the treble, but if you're sensitive to those sorts of things, you may come upon some peaks form time to time. Obviously, if you're up for EQing then you can tune to your preference.

Ideally, you could get both, try them out for a week and return the ones you like less.

In my experience a good way of going about things is:
1. Look at measurements and don't believe golden eared audiofools with fluffy language and ill-defined, inconsistent terminology
2. Using measurements and your budget - narrow down the selection
3. If possible - demo the selection - there's a lot of stuff that matters - comfort for you personally, the build quality, even looks - for example, the AKG371 are a good measuring, affordable closed back - but I just can't use them because for me the fit is not good and I dislike the build intensely
4. Give yourself some time - I found that over a sufficient time span I notice I'm grabbing one piece of gear more often than another and if something is not being used it's time to sell/return - now, if you don't have the luxury of time - you'll be fine since you started out looking at good measurements
5. Dial in your taste - you may prefer more bass, more treble, or less of both - if you start off with good measuring gear you can dial in your taste via EQ - this is the point that most subjectivists can't seem to get - they want "flavour" but they seem to want it in their gear - which is all fine and dandy if you have money to burn and really like spending on gear - just not the way most people should do it.

Obviously, most of the people here like the hardware itself - not just enjoying music - otherwise we wouldn't be spending so much time, effort and money in this hobby - but my advice is for that balancing act to favour listening to music (or enjoying other sound - e.g. movies, games) as much as possible - you'll find you can get good, clean gear for not a lot of money and enjoy your music just fine. Once I started making this thought prominent in my mind - I could more easily sell stuff I wasn't really using (e.g. Focal Clears, and all but one AMP and DAC), and I feel like I don't have to chase the next great thing. Some of us learned by making mistakes and starting off as audiofools - probably best to learn from our mistakes rather than making your own

Beautiful comment, I really appreciate the advice!

 

[shaney777]

Doesn't matter much which headphone amp you have as long as it's been measured by Amir on this site and has measured well. I have the Atom Amp and HD600 and of course it drives them fine. I think you're wasting your time/money choosing the HD600 though, with the exception being if you know you're not fussed about soundstage & size of soundstage - they've got a small soundstage. HD600 are one of the best headphones for everything else though, apart from the bass which is a bit woolly & ill-defined when boosted. (HD560s as per my last post is the better headphone all-in-all).

I think I don't care much about soundstage. I've enjoyed Grados a long time! I think most important is neutrality and mids.

 

[shaney777]

From what I remember a lot of people tested it with the HD6XX which has the same power reqs as the HD600, and it worked well with no issues. Specs also indicate that it shouldn't be an issue.


More than sufficient for HD560s. It is quite easy to drive.

Thank you! I hear a lot of praise for the Atom. I tried IEMagni as well and preferred the Atom sound signature, although one seemingly knowledgeable member on some forum told me that they should sound the same. In my experience, Atom produced incredibly airy, impactful bass, which IEMagni sounded relatively boring. No idea.

 

[Eldgrim]

Thank you! I hear a lot of praise for the Atom. I tried IEMagni as well and preferred the Atom sound signature, although one seemingly knowledgeable member on some forum told me that they should sound the same. In my experience, Atom produced incredibly airy, impactful bass, which IEMagni sounded relatively boring. No idea.

Sounds like the headphones weren't driven properly. Was it with your Hifiman? Many planar magnetic drivers have low resistance and are more dependent on current than voltage to be driven properly. Could be that the IEMagni simply does not output enough current for your headphone while still being transparent in its signal processing.

 

[Robbo99999]

I think I don't care much about soundstage. I've enjoyed Grados a long time! I think most important is neutrality and mids.

Ok, if you know you don't care about soundstage, then HD600 could be your best headphone, especially if you're limiting yourself to not using EQ. HD600 sound good with just a small bass boost, Low Shelf at 75Hz at +3dB, they don't really need anything else doing to them. They do have some of the best mids in my experience, so yes go for that HD600 then.

 

[ifloatoveryou]

I would actually recommend the sennheiser 6XX, assuming you're able to order from drop. Amir couldn't tell the difference between the 600 and 650, they have a very similar sound signature, and 6XX is the same as the 650 soundwise. The 600, 650 and 6XX all will meet your criteria of neutrality/mids, I only recommend the 6XX because they are cheaper.

 

[shaney777]

Thank you all. I have decided to go for the HD 600. My problem now is that it seems Hifiman is giving me the run around about getting a replacement. It plainly says on their website that I should be able to get a repair or replacement, but the email service rep seems to be claiming that I do not get a replacement unless I request a headphone that costs significantly more than I paid. I will be calling them tomorrow to deal with this...

 

[Jose Hidalgo]

I really appreciate the breakdown. How do you feel about the HD-600 vs. Sundara in terms of sound quality? These are going to be the same price, and one is my dream headphone.

OK, I have to answer this. I've been owning both the HD 600 and the Sundara for almost a year now. Some of the previous comments don't take EQ into account. I do, because to me it changes everything for these two headphones (for any headphone actually). So nevermind all the non-EQed frequency plots, really. In this post, let's just assume that I'm talking about EQed headphones, unless I specifically say otherwise.

Both the HD 600 and the Sundara lack bass without EQ :

Spoiler: Without EQ

• HD 600 : roll-off below 110 Hz, reaching 58 Hz @ -3 dB
• Sundara : roll-off below 140 Hz, reaching 54 Hz @ -3 dB

A simple EQ via Oratory presets (included with my app) greatly enhances both of them :

Spoiler: With EQ

• EQed HD 600 : reaches 42,5 Hz @ 0 dB, and 30 Hz @ -2 dB, with a preamp gain of -5.5 dB
• EQed Sundara : reaches 63 Hz @ 0 dB, and 35 Hz @ -2 dB, with a preamp gain of -5.5 dB

But we can try to push things a little further.

Spoiler: Here's the juicy stuff

• For the HD 600, it can be a challenge to push bass even further, because of the risk of distortion, and even of driver degradation if you really listen at them very loudly. I would advise a bit of caution here. However I have successfully added an additional filter to their Oratory preset, with this simple EAPO code that anybody can try :
  GraphicEQ: 20 2.3; 30 2.3; 42 0; 20000 0
  The HD 600 then are able to reach 30 Hz @ 0 dB and 20 Hz @ -1.8 dB. Of course preamp gain needs to be adjusted from -5.5 dB to -7.8 dB.
  I was expecting distortion, but to my surprise I got nothing really disturbing. Only the subjective impression that we are reaching the limits of this headphone, which is normal after all. I guess it depends on how loud you listen to them, so if you remain reasonable, everything should be OK.
  
  
• For the Sundara, like Oratory states in his PDF, the 28 Hz parametric filter gain can be changed from 0.0 (no bass extension) to 7.0 dB (linear bass extension). In my experience, 2.0 dB seems like a good balance, plus this value was already used in Oratory's preliminary Sundara preset. This makes the Sundara reach 30 Hz @ 0 dB and 23 Hz @ -2 dB. Preamp gain needs to be adjusted accordingly, from -5.5 dB to -7.1 dB.

All this comes at a cost : greater preamp gains mean lower output volumes and possibly the need of a bigger amp.

But in my case it's just fine. I can drive both these headphones with my Topping L30 in medium (0 dB) position and volume knob always in the max position (as it should be according to John Yang, one of the L30 designers - I only use digital volume control within my player). And if I really want to crank it up, I can always put my L30 in high (+9 dB) position, which gives me 9 dB of headroom. That's more than I need with any of these headphones, so I'm perfectly happy. The L30 is such a tiny little beast, it's amazing !

Now on to the big question : HD 600 or Sundara ? which one do I prefer ?

Spoiler: Here's your answer !

... quite honestly, after almost a year of use, I would go for the HD 600. Here's why :

• First, the HD 600 are more comfortable than the Sundara. Some may think that comfort should be the last criteria. I too thought so some time ago. But now I have to digress. Comfort is of the utmost importance so your mind can really focus on music.
  --> The HD 600 are not "extremely" comfortable : they're basic, have a bit of clamping force and lack a proper suspension system (after all they're studio grade material, they need to be simple and unbreakable, lol). Both of my Nighthawks are infinitely more comfortable.
  --> But the Sundara are more uncomfortable than the HD 600, that's for sure. They're clearly heavier (372g vs. 260g) and feel so on your head. They're always "there". They're not suited for big heads or big ears (round pads), whereas the HD 600 are just fine in either case.
  
  
• Both the Sundara and the HD 600 are open back, which is a good thing because you don't get any internal reflections / resonances. The sound is clear and precise without any unwanted echo (my Nighthawks have a bit of that echo, which is bad on most songs and good on some of them - a nice side effect I guess).
  
  
• None of them (HD 600 or Sundara) can make it down to 20 Hz comfortably. The Nighthawks rule in that area, they just have no limits and no distortion so you get all that nice rumble to your ears. It's all there, and you can see that the drivers are comfortable with that, they don't even suffer. Of course when you get to the midrange and treble, they don't sound as good as the HD 600 or the Sundara. They sound "good" but not "natural". For example the guitars don't sound exactly like they should, where as with the HD 600 you get exactly the sound you're supposed to get.
  
  
• As for the soundstage, it's true : the HD 600 have a small, kind of "in your head" soundstage. It can be a problem to some people. To me it's not. It's actually one of the reasons why I wanted them. I wanted that intimate soundstage. But if I want to make it considerably bigger, I just press the "HeSuVi" button in my app, and voilà. Works beautifully with the HD 600.
  
  
• All in all, the Sundara sound is really nice, but is it nicer than the HD 600's ? After some extensive listening, I don't think so. The HD 600 bring that "magic touch" to guitars, strings and vocals. I can't explain how they do it, but the Sundara don't do quite as well. The Sundara are great, but the HD 600 are magic to my ears. I would have expected for the planar magnetic technology to make a little difference here, but to me it doesn't (so close though) and it just weights heavier.
  

To sum it up : comfort = HD 600 / Open back = both / Sound = HD 600 (but really close) / Sub-bass = none of them, but with EQ you can get down to decent levels, with no clear winner here / Soundstage = Sundara (but bigger doesn't necessarily mean better, it's a matter of taste).

So don't get me wrong : the Sundara are really great and probably worth their price. I just think that the HD 600 are even greater. It's amazing what Sennheiser was able to achieve back in 1997 (they haven't been modified since then, only aesthetically restyled). The HD 650 couldn't do better (only slightly different), and neither could the HD 660S. And now thanks to EQ we get to enjoy the HD 600 in all their glory, and it really makes a big difference on some pieces of music.

What an amazing pair of cans. @shaney777 , you are going to love them so much. Long live the king !

 

[RHO]

OK, I have to answer this. I've been owning both the HD 600 and the Sundara for almost a year now. Some of the previous comments don't take EQ into account. I do, because to me it changes everything for these two headphones (for any headphone actually). So nevermind all the non-EQed frequency plots, really. In this post, let's just assume that I'm talking about EQed headphones, unless I specifically say otherwise.

Both the HD 600 and the Sundara lack bass without EQ :

Spoiler: Without EQ

• HD 600 : roll-off below 110 Hz, reaching 58 Hz @ -3 dB
• Sundara : roll-off below 140 Hz, reaching 54 Hz @ -3 dB

A simple EQ via Oratory presets (included with my app) greatly enhances both of them :

Spoiler: With EQ

• EQed HD 600 : reaches 42,5 Hz @ 0 dB, and 30 Hz @ -2 dB, with a preamp gain of -5.5 dB
• EQed Sundara : reaches 63 Hz @ 0 dB, and 35 Hz @ -2 dB, with a preamp gain of -5.5 dB

But we can try to push things a little further.

Spoiler: Here's the juicy stuff

• For the HD 600, it can be a challenge to push bass even further, because of the risk of distortion, and even of driver degradation if you really listen at them very loudly. I would advise a bit of caution here. However I have successfully added an additional filter to their Oratory preset, with this simple EAPO code that anybody can try :
  GraphicEQ: 20 2.3; 30 2.3; 42 0; 20000 0
  The HD 600 then are able to reach 30 Hz @ 0 dB and 20 Hz @ -1.8 dB. Of course preamp gain needs to be adjusted from -5.5 dB to -7.8 dB.
  I was expecting distortion, but to my surprise I got nothing really disturbing. Only the subjective impression that we are reaching the limits of this headphone, which is normal after all. I guess it depends on how loud you listen to them, so if you remain reasonable, everything should be OK.
  
  
• For the Sundara, like Oratory states in his PDF, the 28 Hz parametric filter gain can be changed from 0.0 (no bass extension) to 7.0 dB (linear bass extension). In my experience, 2.0 dB seems like a good balance, plus this value was already used in Oratory's preliminary Sundara preset. This makes the Sundara reach 30 Hz @ 0 dB and 23 Hz @ -2 dB. Preamp gain needs to be adjusted accordingly, from -5.5 dB to -7.1 dB.

All this comes at a cost : greater preamp gains mean lower output volumes and possibly the need of a bigger amp.

But in my case it's just fine. I can drive both these headphones with my Topping L30 in medium (0 dB) position and volume knob always in the max position (as it should be according to John Yang, one of the L30 designers - I only use digital volume control within my player). And if I really want to crank it up, I can always put my L30 in high (+9 dB) position, which gives me 9 dB of headroom. That's more than I need with any of these headphones, so I'm perfectly happy. The L30 is such a tiny little beast, it's amazing !

Now on to the big question : HD 600 or Sundara ? which one do I prefer ?

Spoiler: Here's your answer !

... quite honestly, after almost a year of use, I would go for the HD 600. Here's why :

• First, the HD 600 are more comfortable than the Sundara. Some may think that comfort should be the last criteria. I too thought so some time ago. But now I have to digress. Comfort is of the utmost importance so your mind can really focus on music.
  --> The HD 600 are not "extremely" comfortable : they're basic, have a bit of clamping force and lack a proper suspension system (after all they're studio grade material, they need to be simple and unbreakable, lol). Both of my Nighthawks are infinitely more comfortable.
  --> But the Sundara are more uncomfortable than the HD 600, that's for sure. They're clearly heavier (372g vs. 260g) and feel so on your head. They're always "there". They're not suited for big heads or big ears (round pads), whereas the HD 600 are just fine in either case.
  
  
• Both the Sundara and the HD 600 are open back, which is a good thing because you don't get any internal reflections / resonances. The sound is clear and precise without any unwanted echo (my Nighthawks have a bit of that echo, which is bad on most songs and good on some of them - a nice side effect I guess).
  
  
• None of them (HD 600 or Sundara) can make it down to 20 Hz comfortably. The Nighthawks rule in that area, they just have no limits and no distortion so you get all that nice rumble to your ears. It's all there, and you can see that the drivers are comfortable with that, they don't even suffer. Of course when you get to the midrange and treble, they don't sound as good as the HD 600 or the Sundara. They sound "good" but not "natural". For example the guitars don't sound exactly like they should, where as with the HD 600 you get exactly the sound you're supposed to get.
  
  
• As for the soundstage, it's true : the HD 600 have a small, kind of "in your head" soundstage. It can be a problem to some people. To me it's not. It's actually one of the reasons why I wanted them. I wanted that intimate soundstage. But if I want to make it considerably bigger, I just press the "HeSuVi" button in my app, and voilà. Works beautifully with the HD 600.
  
  
• All in all, the Sundara sound is really nice, but is it nicer than the HD 600's ? After some extensive listening, I don't think so. The HD 600 bring that "magic touch" to guitars, strings and vocals. I can't explain how they do it, but the Sundara don't do quite as well. The Sundara are great, but the HD 600 are magic to my ears. I would have expected for the planar magnetic technology to make a little difference here, but to me it doesn't (so close though) and it just weights heavier.

To sum it up : comfort = HD 600 / Open back = both / Sound = HD 600 (but really close) / Sub-bass = none of them, but with EQ you can get down to decent levels, with no clear winner here / Soundstage = Sundara (but bigger doesn't necessarily mean better, it's a matter of taste).

So don't get me wrong : the Sundara are really great and probably worth their price. I just think that the HD 600 are even greater. It's amazing what Sennheiser was able to achieve back in 1997 (they haven't been modified since then, only aesthetically restyled). The HD 650 couldn't do better (only slightly different), and neither could the HD 660S. And now thanks to EQ we get to enjoy the HD 600 in all their glory, and it really makes a big difference on some pieces of music.

What an amazing pair of cans. @shaney777 , you are going to love them so much. Long live the king !

What version are your Sundara's? I have bought my Sundara's very recently and have the HD6XX since last year.
I've tested with slow sweeps and I get good extension down to 20Hz on my head with the Sundara's (no EQ) but not with the HD6XX.
I have a small head so both Sundara and HD6XX are comfortable to me although the Sundara needs to be on the smallest setting and that is borderline for me. Any bigger and the cups would end up too low.
Soundstage on the Sundara is indeed bigger than on the HD6XX (seems to be the same on all HD6xx series headphones).
I would rate the sound on both exactly the opposite as you rate them. Sundara sound more open and lively. That doesn't mean I don't love my HD6XX. It just more closed in, cozy and warm sounding.
My choice: On a budget = HD6XX, can spend more = Sundara