M50 most certainly does benefit from an amp. It mostly it has an effect on the soundstage size, sound clarity and bass control. If you have a larger Mp3 player such as an Iphone, Ipod Classic or Touch, then it's not required.
But..for say an Ipod Shuffle or tiny mp3 player...of course you'll want an amp! The M50 with those tiny players definitely deserves some sort of amping. The E6 or E5 will do the trick. They're dirt cheap. Some people are so against buying amps, but yet they'll buy $300 portable headphones.
BTW if you find the M50 to have recesseds mids, try it with the Nuforce Icon Mobile. Now I'm not sure I could say it has recessed mids with that amp. Or you could just EQ it
Now I don't suggest spending $100 on an amp for one headphone. Some of us own many headphones, so $100 for a decent portable amp is worth it.
One thing I've noticed is that the M50's bass is more bloated out of the Ipod Touch 2G. Out of a more neutral player the sound is more balanced out. This may be why the mids are more recessed sounding on Ipods. Ipods are supposed to be neutral, but not mine! I stopped using mine because of this.