You should also read the thread "Which of these is a Grace Design m903 a goof match for?" including post #23 and #24. Jeff Graw and I seem to disagree with third_eye and Olias of Sunhillow regarding Grace Design m903 + Sennheiser HD 800, but we do agree that Grace Design m903 is a good DAC and headphone amplifier.
You might also like to read post #2 "Brings life and emotion to Sennheiser HD 800" in the thread "Lynx Hilo: Any opinions on the Headphone amplifier of the Lynx Hilo?" where I describe my first impressions based on a demo involving Lynx Hilo, Grace Design m903, Benchmark DAC1 HDR, Sennheiser HD 800, and Sennheiser HD 650. The impressions from the demo have so far held up after I later received my Lynx Hilo.
Be aware that "I listen at (very) moderate loudness levels." as stated in my signature and described in Grace Design m903 display readout numbers in the above mentioned post #23. Higher loudness levels may give different results.