I would like to update this old post with my recent experience with the DT-770 / 600. About three months after the driver replacement, the left phone stopped playing. So I sent them to the authorized Czech service center for repair again, and about three weeks later I had them back, driver replaced (for the second time...). But there was something weird about the sound which I just could not put a finger on. After about an hour of playing with foobar, comparing to other phones etc. I tried inverting the phase of one of the channels (I've got a plugin for foobar which can invert the phase of both or one of the L/R channels). And that fixed the sound! So I opened the left phone (again...) and noticed that the cable is soldered to the driver exactly the same way it was before the last repair, just the driver had the "positive" yellow color mark on the other side... I did not want to wait another month for the service center to fix the polarity, so I re-soldered it myself, switching the left positive and ground connections, leaving the ground going to the right phone soldered to the input cable ground of course, and all is well since then. I just hope I won't have any more problems with those headphones because I think they sound just great, better than my Denon AH-D2000.
I also bought myself the DT-880 / 600-ohm yesterday, and so far I'm enjoying them very much. The only thing that surprised me is that I expected them to have a little bit less aggressive treble than the DT-770, which to me seems to be not true, maybe even the opposite.
None of my headphones come close to sounding as good as the HD-650 though, they are still the king of my collection.