Received mine today, I have to say they changed my perception of headphones. Know that I've NEVER owned any headphones above 100$ other than Sennheisers.
Here are my main impressions:
1) COMFORT! Holy crap why did I ever think HD650's are comfortable?
Let me explain. Ear pads are surprisingly stiff, much stiffer than on HD650's, but Q701 has virtually no clamping force compared to Senns so it doesn't matter that much, although there is a spot on the edges of my jaw which felt under pressure after some time. But headphones are new, so its to be expected they won't fit perfectly at first. I could keep them on my head for longer than an hour without touching them, with HD650's I have to push my eyes back into the sockets every 10 minutes due to the pressure on my head. I can't even feel the headband due to my thick fluffy hair (eat your hearts out bald guys) and they seem to be lighter than HD650's cable. They don't press on my glasses either which is great. Note that these impressions come from having them on my head for about 2 hours.
2) HARD TO DRIVE! Holy crap why did I ever think HD650's are hard to drive?
Running them off the Little Dot Mk5....HD650's are comfortably loud at 40%, very loud at 55%, extremely loud at 70%, try-it-if-you-dare loud at 85%, I never went above.
Q701's are almost inaudible up to 40%, comfortably loud at 55%, very loud at 70%, extremely loud at 90%. Its no problem since amp is silent and there's no hiss even at 100% and I don't mind having to crank it, but still, I am seriously surprised that they're harder to run out of a solid state amp. I'd get it if it was a OTL with higher output into high impedance headphones, but Little Dot Mk5 should have considerably more output wattage into 62Ohm than into 300 Ohm headphones, especially since they have virtually identical sensitivity, 105db vs. 103db.
Now, running them out of Asus Xonar ST integrated amp its a different story. Card seems to have an abundance of power with both headphones. On low gain, which is useless with any headphones I've ever tried, its too silent. But used in middle 64-300 Ohm gain, its perfectly comfortable at 30%, very loud at 45%, too loud at 55%, which is basically the same as with HD650's.
3) SOUND QUALITY! Straight out of the box, Q701's sound better to my ears than HD650's with over 150 hours of burn in. Simply crystal clear sound, not as full as HD650's, but not thin either, more natural than HD650. Taking Q701's off and putting HD650's on, its pretty obvious that HD650's are considerably darker sounding and I have to admit somewhat veiled and muddy, but indeed less fatiguing. Sound stage seems to be slighlty bigger on Q701, but still they sound more forward and not distanced like I've read somewhere. Imaging is better, instrument separation is better and they're more detailed, but at the same not fatiguing, although not as smooth as HD650's. Also, they're more fun than HD650's, which is what I was looking for. While HD650's tend to artificially make all the music sound relaxing and smooth, Q701's play it as it is, so soothing music is soothing, aggressive music is aggressive. Mids are more forward than HD650's, again, nicer sounding with vocals. Highs are about slightly more sparkly and clear on Q701's, and obviously, more forward than on 650's.
So, I love the soundstage, imaging, details, mids and highs. In those areas Q701's beat HD650's, in my opinion.
Now....bass...well, I expected the bass to be weak and underwhelming. But, its not weak, or recessed, its pretty good in my opinion considering the burn in time. Bass on HD650 also was much weaker when they were new. Sure it doesn't have the brutal mid-bass impact of HD650's, but it has more than enough to make the music satisfying. Also, its slightly more detailed and controlled, and extends deeper I believe, or that is just an illusion since there's no almost 10 db mid bass hump that HD650's have. I actually prefer the Q701's bass for almost all the music apart from dubstep, electronic or techno, which I don't listen to that much anyway, so its not a big deal. I wouldn't like these headphones if those genres were my main listening material. I would say bass from Q701's is similar to Monitor Bookshelf speakers, and HD650's is similar to a bass from a subwoofer.
I could definitely use more bass, but since I have bass heavy headphones, its no big deal.
Another thing that I noticed now, and haven't noticed with HD650 that much, is the difference in sound between Xonar ST and LD Mk5. Most obvious difference is soundstage, which is really massive on Mk5 and quite smaller on ST. Also, sound seems to be a bit fuller and with slightly more bass.
Of course, Q701's are new and don't even have 5 hours of usage time yet, but I like the sound more than HD650's already for my main music genres.
Negatives? Well, they don't look or feel as classy as HD650's. To put it nicely, they look like toys compared to HD650's. Especially with their cheap thin cardboard box and green cables. HD650's come with a sturdy heavily padded case where they can be stored. Q701's have to be exposed to the dangers of dogs and cats. Sturdy plastics that feels almost like glass and shinny lacquer finish on HD650's just looks more expensive. I also would prefer the K701 look, but these are headphones, not fashion objects.
Build quality also isn't as good as on HD650's, but that mainly due to the type of headband. They don't feel as sturdy and high quality. Also, Q701's aren't as resistant to vibrations as HD650's. When playing music louder, and if I touch any part of headphones, I feel vibrations. With HD650's, the headphones are completely inert, no vibrations at all.
In terms of sound, I may have heard some sibilance on some records that aren't that well mastered, and they do sound thin in some music, but that's only because I'm so used to the HD650's full and thick sound, and I expect both problems to go away after a 100-150 hours of burn in.
All in all, I'd say I'm happy with my purchase so far.