foreverzer0
100+ Head-Fier
One of these factors is output impedance. The HA-501 has a damping factor knob, which adjusts the amp's OI for use with a specific headphone. It could have been set really high when you tested it. Just guessing here...
However, the hd650 he tested it on is fairly high already. Plus wouldn't OI more so affect the frequency response versus volume/power?