I replaced the op-amps in the O2 a few days ago with the OPA2227. Expected it to sound mostly the same, but I don't think I like it.
It actually now sounds too warm, almost muddy sounding with my HD-650 and DJ100. The OPA2134 I seem to love (in my other amps) and I should have went with that.
Seems impossible. It's actually hard to believe this is an O2.
I will instead try the LM4652 and see how that goes. OPA627 sounded like a fun experiment, but too much trouble...