BK Electronics have now released a portable headphone amp, see here. I'm using one of these at the moment to drive my Sennheiser HD570 cans (known for only being their best when supplied with some juice) and it does a great job. I'm mainly using it with the headphone output from a Sony mini-system and it really is awesome - better treble, better bass, more gain and overall improved dynamics. Basically all of the things that I'd expect a headphone amp to provide bearing in mind I havn't heard any others yet... It's basically just an optimised Cmoy and is well worth a look if you want to dabble with headphone amps without spending too much. I must be sounding like some kind of broken BK record by now.