From the OP: "There even seems to be some distortion with highs for some bizarre reason, albeit very minor, even though they're supposed to have great range."
The OP isn't the only one to notice that. There's a weird resonance in the upper mids/lower treble area that you only hear on some material, but I've noticed it as well, and another forum member commented on it extensively. 'Pure vocals' is a very subjective judgement unless you've heard the particular singer in question live. A vocalist will sound very different out of the k70X than they do out of the hd580. Which one is more 'pure' is anyone's guess. Preference in this hobby is very subjective.
I don't think this plasticy resonance exists on the Q701. I certainly can't hear it. It was there on the K701 for me, but not the K702 or Q701.
BTW the Q701 to my ears with any amp is so different than the K701 that vocals sounds just as good as the HD-580, if not better. They're actually quite close. The 580 isn't quite as warm sounding as the HD-600 though and has less mid-bass and more forward upper mids IMO. The Q701 I have is much fuller and warmer sounding than the K702, so it's much closer to the Sennheiser sound in many ways. Not like it, but closer than before. The Q701 no longer has those lean mids and analytical sound to my ears. I can get that result out of any decent amp, including my Airhead. My old K702 almost felt like it had some recession in the mids somewhere. I never ever get that with the Q701. The Q701 never sounds thin or lacking in the mids. Sure, compared to an HD-650!
Most likely even for you, issues you had on the K702 are not going to be present on the Q701. The treble is much easier on the ears too.
For me they made the soundstage much more accurate. For those that find the Q701 bad for vocals (I don't and not even remotely), wait until you try the K701!
Previously I preferred the K601 for vocals over the K702, but the Q701 has it beat. Vocals should never sound distant unless it's the recording or it's bad amp/dac synergy.