If you want good recommendations then you're going to need to provide a bit more information.
First, are you looking for an amp/dac or only an amp? If you're currently running an o2+odac from a reputable company that combo is good enough to run pretty much any pair of headphones you could possibly want until you start getting into either very expensive sets ($2000+) or specific types of audio styles (which is almost entirely subjective to your tastes).
Additional information we need would be:
- What kind of headphones are you considering in the future and on what kind of budget?
- Do you like open or closed headphones?
- Do you like/dislike tubes?
And so forth... If you're looking for a straight upgrade then something like the Grace Design M9XX is a pretty reasonable amp/dac combo with some very high end features for right around $500 that will easily run anything you throw at it. I've got some Sennheiser HD650s, AKG K7XX, and some Fostex TH-X00 headphones that all sound great on it.
With that being said if you intend to get some HD800S or some Audeze LCD-4 headphones then you're probably going to want to do some more research and pair those with an amp that is complementary to the specific pair you pick out. Which, a purchase like that should be done after having heard them in person at a good audio store... not over the internet.