May I suggest a different headphone?
Try an AKG K240 Studio. Much better than the M50 IMHO, and it'll gain much more from amping than the M50. It's also about $50 cheaper. In my experience the M50s actually sound worse amped because it brings out the mids... which are atrocious.
The highs on the K240 Studio are absolutely stunning, and the overall sound is very warm and laid-back. The M50 has sharp highs that smack you across the face and the overall sound isn't particularly well-detailed or textured.
The M50 works with everything but classical and jazz if I remember correctly, and the K240 S works well with everything I've tried. The K240 is also EXTREMELY comfortable, AKG makes the most comfortable headphones ever. One thing to note is that the K240 is semi-open/vented, so not AS good isolation as the M50. However, I've found that it is still pretty good at attenuating outside noise provided there's not a concert going on.
Just something to think about. You know you're an audiophile when you try to give others advice on the headphones they should buy, audiophile or otherwise.