Forgive me if this sounds a bit incoherent, I haven't had a lot of sleep and I've been pounding away at algebra homework all day, so there isn't much left at this point, but here goes.
I'm writing this review with the intention of comparing these two headphones, considering I have seen several people view the Q701 (and K701/2) as an "upgrade" to the AD700, I figured I'd list my thoughts concerning this.
Put quite simply, the AD900 sounds like a sonically "correct" version of the AD700. It shares the same signature sound, soundstage, and ability to resolve many quiet nuances with ease. Anybody who has ever noticed (and been bothered by) the weird holes in the mid-range the AD700 has will be pleasantly surprised with the AD900, as this is the major difference between the two.
The Q701 is very "flat" sounding. I wouldn't call it entirely accurate, (even though it is a very neutral headphone), but even with it's fairly neutral sound, it feels like something is missing, especially in the midrange, it sounds somewhat recessed, the AD900 sounds very up-front and lively. The Q701 has much better bass.
I'm gonna make the remainder of this review a comparison of certain aspects of both headphones, since that seems to make the most sense at this point.
Soundstage: AD900 wins here, hands down. It retains the same spatial, open, "front row" sound stage, the Q701 sounds pretty typical as a headphone when it comes to soundstage, it seems to me the angling, size, and proximity to the ear of the drivers in the AD900 does something to open everything up and just make it all feel alive and "real". Q701 just doesn't do that.
Speed (transient response): Again, AD900 wins here hands down, i don't care how congested the sound becomes, everything is rendered perfectly with flawless instrument separation. Don't get me wrong, the Q701 does well here too, but not anything like the AD900.
Accuracy: Honestly, it's hard to say which is more accurate, the Q701 seems to win here but only by a small margin, and then again, there is just something "off" about the mid-range on it, though i can't quite pinpoint it. Both seem very accurate though. The AD900's tonality sounds more realistic and natural in my opinon though (due to better mids).
Bass response: Q701 wins here, simply because the bass itself is thicker. As far as whether or not the AD900 has more/better bass than the AD700, well the answer is both yes and no. The AD900 goes deeper, but is still lean sounding like the AD700. The AD900 does seem to do a better job in rendering the bass compared to the Q701, simply because of it's superior transient response.
Comfort: Comfort has never been an issue for me, i can wear uncomfortable phones for long periods and not really care much about it, but in my experience, the Q701's are probably more likely to be more comfortable (provided you have a good amount of hair) because the pads almost seem too thin on the AD900, causing the driver casing to press slightly against the back of the ear. The Q701's ridged headband would probably bother someone with very short hair.
Detail resolution: As i mentioned before, AD900 reigns supreme. I hear all kinds of textures and nuances i've never heard in the Q701. The Q701 sounded almost veiled in comparison. AD900 also does a better job at this than the AD700.
A couple of thing regarding a comparison between the AD700 and AD900:
The sibilance has been eliminated in the AD900
The nasty midrange gap has been filled
The bass extends down further, but it is no thicker sounding than the AD700
They are significantly less ridiculous looking :P
That's all i can think of for now. Hopefully someone will find this useful when considering an AD700 upgrade (with these two phones in consideration).
Edited by yepimonfire - 10/6/12 at 9:09pm