What about the 'Pure Class A Dynamic Headphone Amplifier' DIY project described on the HeadWize Library Projects page? I haven't used it, but just a thought. I'm using an older Audio Alchemy HPA-1.0 and have been thinking of trying something else with my Grado Sr80s.
A high-current output amplifier for low-impedance dynamic headphones such as the Grados. The amplifier features an new servo biasing circuit which operates entirely outside the audio feedback loop. The author includes a balanced-bridge version of the amp for headphones that are wired in dual-mono as well as instructions for converting a Grado SR80 to dual-mono operation. The bridged amp will output 4 times the power of the standard amp.