If you love how the stock O2 sounds, then there is no real reason to swap op-amps.
I actually only did it because I don't love how it sounds with the Q701 and some brighter headphones.In this case I don't care about having the best measurements. I only care about what my ears tell me. Some minor coloration is OK.
I also found myself almost never using the O2 and op-amps (in most cases) are dirt cheap!
Swapping op-amps on the O2 is easy and all it requires is a few minutes. You don't even need to buy a chip puller. You can use a toothpick(!) or some pliers (not suggested).
So far i've tried the OPA2227 and LM4562. OPA2227 is nice, but seems better for the Q701 than HD-650. It seems to be a little warmer than the stock op-amps.
The LM4562 is a no go with the stock Q701, but I love it with the modded Q701 (which is warmer/bassier). Most say it's a brighter op-amp.
The differences are minor really. For something like a DT-990 or grado SR-325i swapping to the OPA2227 or OPA1234 might be a good idea.
I think you'll probably void your warranty though.
BTW I think the stock O2 is great with warmer headphones like my HD-650 and DJ100. With my O2, the HD-650 sounds exactly how it should without much change at all.
Haha, if you're nuts you could try getting the OPA627 to work with the O2. I don't think it's possible. I think that must be one of the most expensive op-amp there is. It's only single channel and you'd probably need one of those adapters. I don't even know if those fit on the O2. $34 for a single channel op-amp chip!!
The OPA2134s are just $4.50 each.
One i'd like to try is the AD8397. Not sure if it'd work and it'd require soldering onto an adapter.
I'm new to op-amp swapping and it's kind of a cheap & fun experiment.
Edited by tdockweiler - 4/24/14 at 2:42pm