I would just say that as with any TOTL headphone, the better the amp (and DAC), the better the sound. The Ether C is reasonably efficient -- even an iPhone drives it using max volume, and a good portable amp can do a very good job. Do mean the Fostex HPP1? If so, I had one and it is a very good portable DAC and amp -- should be fine with the Ether C. For a portable DAC/amp I'm using a Chord Mojo and the sound with the Ether C is amazing, with enough power to play so loud you would damage your brain let alone your ears.