bigshot
Headphoneus Supremus
I have apple computers and players, so I don't have an amp at all. No point. I'm afraid that I'm not familiar with the O2, but I would bet that if I did some controlled A/B testing, the added expense of a midrange amp over an entry level probably wouldn't make any difference. For a headphone amp, if I needed one, I'd probably use an altoids cmoy.