Sorry for all the multiple posts but...
I was watching a bunch of movies the other day with lots of low bass and special effects etc. I compared the HD-650 with the modded Q701 and used my main setup which drives both with ease.
Overall I'd say they're about equal now as headphones. Of course it depends on preference but I'd be OK with either. Modded Q701 does have a slight edge.
The main difference is that the HD-650 sounded just very very slightly fuller/warmer, but of course sounds more closed in, a little less detailed and not as spacious sounding. It's probably a lot to do with the HD-650 having slightly less treble (but barely).
HD-650 has just a tiny bit more low bass presence than my modded Q701. I'd say like less than 5% for sure. It's very subtle.
Strangely enough I've found the HD-650 less picky with amps. My HD-650 is perfectly good with the O2/Magni to my ears, but the Q701...not so much. On the O2 it sounds a little thinner than it should, but still OK. The HD-650 sounds very similar on my Micro Amp as the O2. Somehow the bass isn't so great on the O2.
Interestingly the Vali and the Micro with the modded Q701 sound extremely close in sound. Unfortunately the Vali sounded bad with everything else..
BTW I bought a Bravo V2 hybrid tube amp to play around with. It will either be pretty decent or just some cheap toy to play around with. I got it for under $60 with next day delivery. Not bad!