Received the DT 770 (80) earlier today and have been listening to them for only 90mins or so.
Comfort wise they're a big step up from the Senn HD438 and I find the ear cups way better too.The isolation is pretty impressive as is the soundstage for a closed back set of cans.
My JDS Cmoy isn't here yet (I pray to Crom they don't get hit by customs), so haven't listened to them with an amp, just my walkman (will test them through my a/v receiver too).
The sound and soundstage (for closed back) is immense! So so good.
Interestingly, I had read some people posting that the 770s suffered from harsh brightness/sibilance, but my set don't have this. (And I detest harsh sibilance).
Build quality seems to be top notch.
Thank you to HiResDes for the recommendation over the m50s too