I bought a pair of these DT660s at a good price from an *£$%"&"! marketplace seller, the last pair they had, saving £70 on the "Prime" price.
I vaguely recall having read somewhere that these are like a closed-back HD650 but would disagree strongly - they are not dark, neither are they in any way veiled or powerfully bassy - to my admittedly imperfect ears they sound more like a closed-back K702, but far more "human"!
The soundstage is excellent for a closed-back design, in fact it matches many open-back designs!
The remarks about the bass being light are very true - the only time the bass sounds "normal" is when I play CDs through the headphones socket of my integrated amp; through the iPod Classic or Sony MP3 player, with or without the Topping NX1 or FiiO E06, the bass is very light.
This is in my opinion a question of relative sounds; the whole sound picture is very light, bright and airy, so the brain adjusts to the light bass as sounding relatively normal in relation to the treble and mids
My other two HPs, the HD598 and Sony MA900, both have forward mids, like many popular HPs, so I was pleased to hear the neutral midrange (ruler flat on the FR graph) sounding very accurate and "right" in terms of what one would hear at a concert - I had been dreading a DT990-type recessed midrange!!
Overall, I find these cans fun, absorbing and tonally accurate (shame the bass isn't a bit more present), certainly more so than the K702s I auditioned recently which left me completely cold.
David Maher wrote in his flagship review that on first and second hearing he found the 660s very detailed - I still do after listening to them for two weeks solid, and the dynamic range is stunning
I listen to most genres of classical and quite a lot of jazz (Miles, Monk, Mingus, MJQ) and find these cans ideal
My advice - buy at a good price (mine were only £89) before these disappear completely!