OK, so far, I've tried the OPA627AU, these do sound a bit better than the standard OP-AMP of the card, plus they improve the bass characteristics a little bit, IMO.
What other OP-AMPs would you recommend? Maybe the OPA2111SM or even the metal can version of the 627, the OPA627SM?
Btw, I plan on upgrading the headphone-out only. ^^
The OPA2111's are my favorite so far. Since I messed up on my LME49710HA and my LME4720HA I won't have an opinion on them until they get here. The 627 are ok. I'll put one on the sub but I prefer the 2111's on the front and headphones.