It has to be at 32 Ohms. The Audio-gd NFB-10SE puts out 8+ watts at 32 Ohms and has an output of 590 mw @ 600 Ohms. It drives my HE-500s sufficiently but the volume is near maxed out on softer music. The sound is amazing though with very black background, good instrument placement and extreme detail without being analytical.
Edited by deadkenny64 - 10/13/11 at 5:20am