Damn, my friend apparently left his Q701's in LA, so I'll have to hear them another time. He was impressed with the HD650's, though. He told me he found the DT880's to be muddier than the Q701's when he demoed them, but that's with both unamped. No idea how they would compare with amping.
I heard the DT880 our of both the Valhalla and the Little Dot MKiii. They sounded OK. They were a little too muddy, and they lacked treble IMHO. When I used the Q701 on these, the impedence mismatch caused severe bass distortion and it went out of control, so couldn't really compare on the same amp. I really wanted to like a beyer, but I just couldn't justify keeping the DT880's.
Side note, I had to sell my DT770's too due to amp requirement for portable use, and I didn't really want to carry a portable amp. Been there, done that.