Well the HD660S are on top of my head, been listening for all of two hours, early impressions are good, listened to a variety of music (classical, techno, hip hop, synth pop, Irish traditional). I am listening to FLAC via Foobar 2000 > Modi Multibit > Lyr 2 with 1953 Western Electric JW 2C51 tubes
From memory, first thing that jumps out is the improved clarity, as others have said. This is most definitely an HD6X0 headphone, tonally I could not tell them apart from the HD650 without having them side-by-side (which, as I said before, I sold about two weeks ago). There isn't much else to say there, as the tonal profile is so similar to the HD650. I don't think the bass reaches deeper, but it does sound more refined, likely aided by the improvement in clarity. I've never felt the HD650s are "veiled", but to those that do, the HD660s and the improvement in clarity will be to your liking

the head-space on these headphones is very similar if not the same as the HD650s, indistinguishable to my ears. Again, minute differences in staging and depth cannot be discerned without a side-by-side comparison. Overall I am happy, although I knew I would be; I am not extremely hard to please, and knowing I enjoyed the HD650 going in, any improvement would be a plus. It's an HD650 with improved clarity and more refined bass, that is as much as I can say at the moment.
Clamping force out of the box is very high, already did a little bit of stretching to relieve it. Hard to say if it is just because they are brand new, but the new pads feel stiffer to me, which hopefully means they are more durable. They do feel roomier, we'll see if that changes as they inevitably get more smushed.
It will be up to ya'll to decide if these improvements are worth the additional $200-300. I can't see a reason to own both, though, they are too similar even if the HD660S is improved. If you have the disposable income and want an upgrade from the HD650, they are definitely that.
Cheers!
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