Just received my O2 to connect to my dac-x6 last week. Both set at low gain, O2 at 2.5x.
Quick impression comparing x6 and O2:
O2 has better bass definition and better instrument separation, while mids and high are almost identical. Will try comparing again after listening to O2 for longer period of time.
As it stands, then X6 has damn good amp and amp rolling will really improve.
O2 very good for this price, more powerful than x6 at low gain.