O2 is a good match for the Q701 and I love mine. I use it now in my bedroom for my blu-ray players using the E17 as a DAC.
I do think the Headroom Micro Amp is better overall but it costs like $350 when I bought it. I would say that they're so similar in sound that I'm never going to A/B them too much.
Micro just seems 97% like the O2 but with slightly smoother treble (not rolled off at all). O2 might be like 1% clearer, but maybe not.
I've yet to hear anything else so similar in sound to the Micro. Micro is totally underrated for the Q701 and I think that won't ever change since it's supposed to be discontinued.
Only problem with O2 is that it's obviously totally unforgiving with crap/harsh recordings. I'm even going to say that perhaps they sound even worse, but only slightly.
So far for the Q701 I'd rank my amp preferences as:
Micro Amp > E9 | O2 > Magni > Asgard1
It's hard to say anything bad about the E9 since it makes the Q701 sounds even better. Adds a tiny bit of warmth and sounds pretty smooth. It's not as transparent as the O2 obviously.
Strangely my old Total Airhead is pretty good with the Q701. I don't know where the E17 stands since I rarely use that for other than a DAC.
I wonder if the O2 has any similarities to the M-Stage? I imagine they're pretty close but with the M-Stage having a barely audible touch of warmth perhaps.