I have owned Fostex TH-X00, AKG 7XX and O2 amp/DAC since last year, plus I have changed better quality USB cable and special side ordered the original adapter for the O2.
So till now, already more than a month after listened and tested, (just my personal true experience and thought without related with any other website forum like head fi), O2 power is good enough to drive AKG 7XX and Fostex TH-00X however the sound is not really what I like.
The best about O2 is the background 100% silence (without any distortion, ground hiss and noisy hump), provides natural clean sound and clear vocal, however not really suitable for the bass lover like me, hardly hear any punch, tight and real smooth bass.
Beside AKG and Fostex, I also have tried to pair with other good quality branded headphone with O2, so far BMW P7 is the best partner, Sony Z7 and VModa M100 still acceptable and DR.Dre beats studio 2.0 is the worst result from what I have heard.
The Actual of AKG, Fostex, Philip X2 and ATM-50X are famous in beautiful bass headphone, I can hear such a big different when pair with Oppo HA2 portable DAC AMP, just feel disappointment such a waste when use with O2 DAC AMP.
Of course, I don't expect everyone must agree with what I have listened and tested, nothing is perfect in this world, every one has our own opinion or different thought. Hopefully, my experience might be helpful.