Off-topic, but I'm just reading an article that made me think of this. Especially this bit:
This confirms my thoughts on the HD800, that it loves very-low output impedance amps to get the most driver control. If I were you I'd be using the lowest output impedance mode on the amp and bumping the LF up a bit with a good EQ.
It's funny though, the amp Sennheiser has released (named HDVD 800, if I'm not mistaken) has a 40-ish ohm output impedance