I just ran across a GREAT white paper on headphone amps. The latest trend in headphone amps is toward zero ohms source impedance. The paper talks about the drawbacks of buildout resistance, which is electronically a series resistance. I have not had the time to read it yet, but the frequency response curves tell that they're looking at how source impedance affects frequency response.

The discussion with a link is at [Link: proaudiodesignforum.com]. I could not get the first paper to open, though. It bears investigating.