Pros: Nearly everything, CHEAP!, Build Quality
Cons: Slightly lacking very low bass. Don't really care.
Mids are nice and full sounding. Not thin or lean at all. If they are it's due to a poor amp match or supposed to be that way in the recording (yeah really). I've even gotten good results with an O2, E9 and Micro Amp. Not much else yet. They sound natural to me. To me this headphone really excels with female vocals. They're very slightly forward sounding. I would say the low mids are more neutral. They're not as forward as those of the Anniversary and I like that. With some recordings the Q701 can sound nearly as warm and full sounding as my HD-650 (try Buena Vista Social Club). You can also have the Q701 change into an AD700 with some tinny/harsh recordings.
Treble is not harsh unless it's due to a poor recording. Even then it's no real problem. This headphone won't make harsh/very bright recordings any easier on the ears. For that you can try the HD-598 or HD-650 to sort of take the edge off. I actually think the treble sounds neutral despite what some say. It seems smoother than what I heard on the K702.
Bass is fine and there seems to be no large mid-bass hump. Not much bass impact and there is a slight lack of low bass presence (without mod). For this reason I sometimes use my HD-650 or DJ100 for gaming/movies.
I don't find them crystal clear sounding, extremely detailed or super revealing. Sometimes they're even a little forgiving of some lower quality tracks. Just slightly. Even Youtube music and Itunes/Amazon samples sound fine (this varies usually).
I find them neutral sound with no major emphasis in any area. I actually find them more neutral than the HD-600. HD-600 has more mid-bass and this has some extra treble. What's the difference?
Soundstage is what I would call pretty good but not great. It's smaller than the K702 but I actually think this is a good thing. It's not massive, super airy and nothing ever sounds too distant. Imaging is fine and there is no lack of center image for me.
This thing is pretty tough to drive. I hate it with portable players. All of them. It gets good enough volume but then it sounds like poop. I can use my HD-650 from my TV headphone jack and get good results (for short periods of time)..with this I need like 90% volume and its not worth even bothering with. Some may disagree but I could hate this with the wrong setup. It seems to sound OK from my E17 but it's just not the same as my main setup. Best setup i've heard it with is what I'm using now (well duh). O2 is 2nd best. I actually liked it with the E9 a lot and the 10ohm output impedance doesn't change the sound in any way.
NOTE: This is basically my impressions after 2-3 years of use and NOT the modded version. Don't forget, that version is about 25% better than the HD-800
My impressions of this has not changed since day one. It's my 2nd favorite headphone of all time and I'm never getting rid of it. Not even if you gave me a T1!
BTW I'd love a closed headphone that was a clone of this or close. It's NOT the K550! You may need to reduce the soundstage and treble a little but the mids may gain some fullness.