Dusty,
One question about this amp. The volume knob stays about the same with my new AKG K701s as with my Senn 650s. Both have very different impendences, 62 and 300ohms, respectively. Both on are on the high-gain setting (which, to me, sounds more dynamic than low-gain with either headphone). In my experiences with past headamps, the knob really needed to be kicked back a good bit for low Z headphones.
Is this because I've used headamps with different circuit designs like dual opamps with a buffer for voltage gain? It looks like your amp is built around a special opamp which is likely a different beast than I'm used to. Sounds great though! Detailed, but not dry or analytical.