Just curious, is it powerful enough to power the HD6XX through the SE output? Because I am looking to get the HD600 in the future if it sounds good.
Note: I am not 100% sure about this answer but I will give you my views anyway, it may turn out a bit vague. Difficult giving a precise recommendation in this topic.
I have been running my Zen Dac using a power supply all the time. I am not 100% sure how much you loose in power running it USB powered (and possibly in noise/interference if you are unlucky). I believe Zen Dac is having difficulties delivering power fully up to it specs if not running through a power supply. I don't think you need to buy the most expensive 5V power supply from iFi though (thus saving some money).
Anyway, I have been running it user a power supply all the time and I
think it manage to drive the HD 6XX with SE cable but balanced cables will definately give you some extra headroom. I have been running a balanced cable about three weeks. You may get into issues (or closer to the limit) if EQing aggressively though and lowering the pre amp value a lot. Depends a little bit of type of music too, for classical soft music you sometimes need higher volumes.
Several things affecting power on Zen Dac
1. Power supply perhaps giving you slightly more headroom
2. Balanced cables for the headphones giving you more headroom
3. If using Tidal Hifi and running the desktop client in exclusive mode you also get some extra headroom.
It could be better buying the Zen Can instead of potentially expensive balanced cables and continue connecting through SE. At least you should be mentally prepared for this step, try HD 600 only using the Zen Dac first.
Finally, looks like a more powerful amp is great for headphones like HD 600 and HD 6XX though. They simply answer/react well on more power, this is what I read anyway?
I am not sure if my decision was correct. Instead of buying quality balanced cables for my HD6XX it may have been a better move buying the Zen Can instead (a slightly more expensive option though).
With my current setup I do find it is driving the HD6XX well though, good enough. Zen Dac + power supply + HD6XX + balanced headphone cable. I will most probably not get the Zen Can. Trying to relax and use what I have right now.
If/when getting a more powerful amp I may get another product/brand, we will see, I do like the slightly warm Burr-Brown sound though, there is definately a reason why iFi is using this chip. If changing brand it would be really nice if it has a 4.4 mm balanced output though since I recently bought this rather expensive cable from Forza Audioworks for my HD6XX... ;-) I am guessing 4.4 mm will be more common in the future but I am not sure.
Your Hifiman Sundara experiences are very interesting, you are writing they sound great through the Zen Dac (and I'm not surprsied about that). I am really curious about the Sundara but also a bit concerned about the Hifiman quality issues.
Finally: If thinking about the HD 600 you should probably compare it to the new HD 560S first though.