Ok, well I'm still not sure how to describe the DT660, I wish I still had an 831 around to compare because from what I recall it's the closest sound from what I remember.
I'll start by saying they don't pair well with brighter amps.
The bass definately isn't the most impressive feature of these headphones, it's fairly subdued. Detailed and fast, but not particularly visceral.
Some types of music just don't sound right with these headphones - Trance/Dance/Electronica etc just seem to be missing something.
However tracks like Massive Attack's Angel seems to have plenty of impact and resonance. Rock style bass has impact and detail, but not a lot of slam.
I'm not sure if that makes sense btw, it's hard to describe
Midrange is fairly neutral, nicely detailed and not overwhelmed by other frequencies. Not intimate and lush like the DT880 though.
Upper end is quite crisp, not grado type snappy upper end, just a little bright. Lots of detail though.
Overall things sound pretty good, but the DT660 sounds a lot better for some types of music than others. For some rock they're great (The The, Jesus Jones etc) yet for some acoustic rock they aren't so good (Paul Kelly). For some more dancey music they're good (Severed heads, some NIN) but others they aren't (trance such as astral projection).
I'm a little at loss to describe the 660 really and even after extended listening I'm not sure if I like them or not - sometimes I love what I'm hearing, other times I'd much rather be listening to something else!
Also, I have to say I've got a bit of headphone fatigue at the moment, I've been testing out AKG, Alessandro and new Beyers so everything's starting to blur together a little
One last thing - I mentioned the DT660 being fairly comfortable despite the weight - after a few hours they really do start to get uncomfy, mainly on the top of my head. Going back to my lighter DT531s is bliss!
Forgive me for a not-so-informative post, it's a lot easier when I really, really like a pair of headphones
added: ok, so I stopped listening to the DT660 and went back to the DT531 - the puzzled frown the DT660 was causing has instantly been replaced by a smile - not only do the DT531 feel like a comfy pair of slippers for my ears, they're the audio equivalent