M50s do not need an amp and sound great with high bitrate music- most of my stuff is ripped to 320 or higher. I listen to rock, jazz, blues, trance and classical- all sound really good through these phones.
I will throw another recommendation out for the M50s- great sounding phones unamped (iPod Classic- 80gb & iPhone 3GS)- punchy tight bass, very good mids and treble. I got them for $85 shipped off of eBay. Can't beat that!