I found the DT770 treble to be a little harsh and prominent when I first got them. Now, with about 200 hours on them, the sound has evened out, either in actuality or in my head. They are still bright, but no harshness or sibilance. I agree with the earlier sentiment that the bass is "accurate," or at least flexible. Instead of forcing bass heaviness into the song as more bassy cans can, the beyers bass presence varies with the recording, going really deep and full when required. I suppose mids would be their weakness, but even they are pretty strong.
I am currently driving them with an iBasso D3 and really enjoying them with it. I know they would be even better served by a desktop amp (tubes, seems to be the consensus), but I wonder how much? Because, they sound really good out of the D3.