I have never owned any Sennheiser headphones, but finally got to try the HD650, HD660S, HD800 and HD800S for a fair amount of time, with my Jotunheim DAC/amp. I started with the HD660S and finished with the HD650.
The HD660S is great! It has a nice, satisfying sound signature, good clarity, good imaging and is pretty comfortable. Of the four headphones I tried, it was the most well rounded one. It sounded oddly familiar - maybe like an open Focal Listen. Good bass, too, nicely rounded off towards the lower frequencies instead of falling apart.
The HD650... I don't understand why they are so beloved. They aren't just laid back, they seemed downright lazy to me, overly softening transients, etc. Maybe there was something wrong with them, or the cable. But they also look dated and feel flimsy. I would actually prefer my Philips SHP9500S over them.
The HD800/S have a much wider sound stage, of course, are more comfortable, more resolving, etc. The HD800 have their treble issues, but I found them more engaging than the HD800S.