Right this moment, I am listening to ER6s plugged into a Go Vibe. I've still got the AD8066 in there that Norm recommended and preinstalled although I also had him send me an OPA2227. I'm enjoying the AD8066 and I've been reluctant to roll it out so far so as to get a real good feel for the amp and the AD8066. I've also been doing some comparisons with a Pocket Amp I also have so that's been keeping my tendency towards audio geekiness active.
I've been asking the same question and I've been compiling a mental list of suggestions from Norm and some other headfi'ers. The AD8260 (which needs a Brown Dog adapter like the AD8066) seems to have a devout following. At $16 or so, its one of the more expensive opamps, but you only need one in the Go Vibe and that's inexpensive as far as audio upgrades go.
A few others that came up in the thread amongt others were the OPA2107, OPA2132, OPA2134, and AD863. Those alone would keep me busy for quite some time. There are hundreds of opamps, some more suitable to audio than others. For certain, my investment in the Go Vibe can potentially go a long way with these and some other opamp recommendations not to mention opamps yet to be on the market. I, too welcome the ongoing recommendations from this group.