I've been looking at the O2 and reading some more reviews, and with a power cord (so that I don't have to change batteries every few hours) it puts my cost at about $150 for the amp. At this price I could easily buy a used Asgard, which has much more power and would likely drive the Q701 very easily. I would like to stay closer to $100, and I've heard the Vali goes very well with the Q701. It has a higher output impedance (6.5 ohms), but the Q701's impedance is 62 ohms, so it is more than 8 times the output impedance. The Vali also has slightly more power than the O2, so if the O2 has no problem driving the Q701, the Vali shouldn't have a problem. At the moment, I think I've decided upon the Q701 with either a Vali or Asgard 1. If I can find an Asgard 1 under/around $125 I'll go for the Asgard, but if not I'll likely go for the Vali.