I've read multiple times, but can't verify personally from theory or practice, that the ideal output impedance of an amp is 1/10 or less of the input impedance of the headphone.
Edit: I just read another technical discussion of headphone amps. Some few engineering details with which I'm familiar were correct. He (or she) says the guideline is 1/8 (not 1/10) and this guideline becomes less important as the ability of the amp to deliver clean power at higher volumes increases.
There's a lot of uninformed regurgitation of engineering terms among some 'audiophiles'. I have 10 hours of (undergraduate) EE, which is enough to recognize some obvious nonsense, but I'll never be an engineer. However, some people are convinced of the 1/10 guideline, so I wrote that a 5-Ohm output impedance is a negative. It might have been better to say "possible negative".
Until I buy another set, I'll be using the M-Stage with 32-Ohm headphones. Maybe I'll be disappointed.
Edited by HamilcarBarca - 8/28/12 at 6:19pm