wait wait wait, I just read this:
http://www.headphone.com/learning-center/how-do-i-know-if-my-headphones-need-an-amp.php
should I invest in a headphone amplifier? where can i test them with my current set?
The m50 doesn't need amplification; they'll benefit from one, but they don't need an amp to perform. If you still want an entry level DAC/headphone amp (it'll increase the clarity somewhat and add some oomph to the bass), it'll set you back ~$100, these are good starting points:
Fiio E10 (USB connectivity, Laptop & Mac friendly) or
Auzentech X-Fi Bravura (PCIe connectivity, tower friendly)
Amplification is a tricky thing, and can cost more than the headphones sometimes. You want to make sure your music is up to scratch (e.g. bitrate), your source is up to scratch (DAC/sound card, etc) and your headphones benefit from amplification (not your average DJ headphones). Most Audio-Technica headphones don't benefit much from amplification, so it makes more sense to spend money on upgrading the source (DAC) rather than pumping up the power. Beyerdynamic on the other hand is very power hungry, and you must have a decent amp before you think about getting a DAC. Each brand and each pair of headphones have their own flavour, you need to know what has good synergy with others. That kind of stuff.
Also, lurk head-fi.org.