I run my M50s through an E7 at work. It's a small, cheap DAC/amp, so you get what you pay for, but anything is better than my laptop's soundcard DAC. It has a nice, simple interface and it's about the size of an iPod Classic (a bit thicker). The sound is probably the best you're going to do at this price range (I paid $50 used), but it doesn't come close when compared to something like the HRT Music Streamer II, IMO. Know that there are good amps available at almost any price range. E7s are among the much cheaper DAC/amps.
These two are the only portable amps I have heard, though, to be fair. I'd look into something less portable if you really want to boost your sound quality.