I am forced to chime in with my opinion on the Earmax Pro. I had a Headroom Max (SS amp) and the Earmax Pro is simply superior with HD650s--more liquid sound, less fatigue (crossfeed notwithstanding), amazing bass.
Be aware that the Earmax PRO is NOT the same as the Earmax--the Pro version can drive some of the more difficult headphones that the standard Earmax cannot drive effectively.
Be aware that the Earmax PRO is NOT the same as the Earmax--the Pro version can drive some of the more difficult headphones that the standard Earmax cannot drive effectively.