Take a look at some Audio Technica ES7 or ESW9A's. I wouldn't worry too much about a portable amp, at most I would look for someone selling a DIY CMOY. Prices on portable amps can be ridiculous for what they are, as they usually aren't much more than a CMOY in a fancy case.

This is unless you will be spending a lot of time stationary in a quieter environment. Then the whole idea of "portable" changes into "easy to transport"