I have to rspectfully, but completely disagree with Walkgoods post above.
I own the ATH M50s and a RSA P-51 Mustang amp, and it made a DRASTIC imporovement in the sound quality, dynamics, detail, and drive out of my Ipod.
I believe this is for a couple of reasons though :
#1 - The Mustang is known for having VERY tight, dynamic and controlled bass, and the M50s desperately need this, as they can be too bassy.
#2 - The Mustang is known for HIGH power delivery and control for a portable, and drive the hell out of my M-50s.
#3 - The M50s are known for being somewhat warm and laid back in the mids and highs, and the Mustang has gloriously beautiful mids and VERY extended and sweet highs.
So yes, the correct portable WILL make magic with the M50s. The key word here though is "correct".