I have Sennheiser CX200 earbuds and like them. I think they sound nice when plugged in to my rMBP. I listen mostly to metal.
I've had the FiiO E07K USB DAC/amp on my Amazon wish list for ages but never did take the plunge. Think it's worth it given that I'd probably be keeping my existing earbuds for the foreseeable future? I also own Beyerdynamic DT990 (600 ohms) but I probably wouldn't use them on this setup often at all as they're connected to my "static" desktop PC.
I'm open to options other than the E07K so long as it's portable and available from Amazon!