Hi LaurenDean,
I had to face the same choice about 6 months ago. I ended up buying a HD600 and I'm glad I did. I think most people should, provided they have access to EQ.
The HD660S is clearly overpriced. The HD600 is much cheaper, and like 3125b wisely said, it can be EQed
closer to the Harman curve (as shown by Oratory's preference score), so ultimately the EQed result will be 'better' if I dare put it that way.
About the Pros you mentioned, a) simply doesn't matter anymore if you EQ them. Just check the HD600 EQed frequency response : it's really hard to get any closer than that to the actual Harman curve. And b), no, I don't think the HD660S is more resolving than the previous models (I would need scientific evidence to back up that claim, not subjective impressions). So within the Sennheiser range, if you have access to EQ I don't see a reason for getting a 660S. The 600 is better when EQed and cheaper.
With that said, for critical listening I would also consider the Sundara. I own both of them (Sundara and HD600). The HD600 may be subjectively (and marginally) more pleasing for vocal and acoustic stuff, it has that kind of magical midrange texture that's hard to explain. But the Sundara is definitely more versatile if you listen to a lot of music genres, plus when EQed it goes lower than the HD600 without distortion.
- HD600 Pros : price (< 300 €), performance when EQed, magical midrange, weight (only 260g)
- HD600 Cons : doesn't go very low (even when EQed), not very versatile, very small soundstage, comfort is OK but not amazing, plastic build
- Sundara Pros : decent price (300-350 €), performance when EQed (including bass down to 30 Hz), more versatile than the HD600, very precise treble, bigger soundstage
- Sundara Cons : weight (372g, which remains reasonable especially for a planar, and the comfort is OK too, so not a big deal)... well, not a lot of cons actually.
Both will need an amp to drive them to their fullest potential. The HD600 is 300 Ohm, but I have found the Sundara to be more demanding, due to its low sensitivity (94 dB). That's also confirmed by solderdude's Headphone Power Table : the Sundara needs more power than the HD600 to reach the same SPL. But again, with an amp it won't be a problem, both headphones are relatively easy to drive with any decent amp.
After some months of listening, I have to recognize that the Sundara is indeed more versatile than the HD600. So ultimately I would recommend it for critical listening. There's a reason why many consider it the best headphone under 500 €. As for me, I'm very happy to keep both, because I can enjoy the HD600 on vocal and acoustic stuff, and switch to the Sundara for the rest.