Sound quality of the WH1000xM3 (based on my experience after doing my EQ cuts to the bass) is simply fantastic. Just today I was listening to what I think is one of the better albums for gauging sound quality, Dave Matthews Band, Crash and I was just as pleased as possible with the sound quality. The layering and separation of instruments was fabulous, tone and texture sounded as I expected, very natural and yet with just enough signature colour to be engaging and vibrant. The treble is bang on, the cymbal reproduction was just perfect as was, well everything. I have owned that album since it came out in the late 1990s and I have heard it on many systems, speaker and headphones, and I know it well and the xM3 portrayed it as well as I could possibly hope for, and all without a cable. Just last night I was listening to the HD820 powered by speaker taps and I didn't feel it made the music as enjoyable. Yes, it is certainly the more technical and believe me brilliant sounding headphone, but the Sony is just dynamic and engaging, plain and simple.
As always, not when left stock signature, not for me anyway, the bass has to be trimmed in a few places to allow the sound signature and capability of the design to shine through. Left stock I wouldn't even give it a 3 out of 5 for sound quality, personally. I am not saying that the xM3 reproduced everything perfectly, just that how it reproduced things was for me a perfect portrayal given my preferences for a slightly warm and dynamic sound with some weight to notes. I will gladly give up a little air for some meat to the sound, as long as the balance is struck between warmth and accuracy.