Just got my pair of Q701's the other day and I'm really liking them. I was also considering the HD 650's and DT990's but decided the Q701 was going to fit my tastes better, and I think I was right. (I didn't get a chance to listen to the 650's or 990's, so I can't compare. I did get to listen to a pair of HD 600's, but as they weren't amped I don't think it would be a fair comparison either).
I listen to a lot of orchestrated soundtracks, hip-hop, R&B, as well as electronic music and acoustic performances; I think the Q701's strengths are just as good in each genre. Oh and I find them to be very comfortable, though I'm wondering how long it will take for the little cords that maintain tension on the headband to break.
Hey man, congratulations!
Just my opinion, but I found that the HD650s were too dark for me.
I thought the 990's were a real nice 'phone, they have a bit of bass and treble boost.
I have a pair of DT880s and my opinion is that the DT880 or DT990 are a gret complimentary pair to the Q701s.