Great that you like 'em! If you look in the ES10 appreciation thread, some people say that a headphone amp can bring out some extra resolution from the ES10 and make the bass more defined.
I usually use my ES10's unamped and only have experience with FiiO's headphone amps. You could try the cheap E6. But that might not make much difference. I've got the E11 and it's pretty good. You could get a LOD to your iPhone and then plug it into the amp.