To go with the Audio Technica ATH-M35s that I recently received as a gift, I decided to purchase a Fiio E10. It’s a small but powerful AMP/DAC combo capable of powering headphones up to an impedance of 300 ohms.
It costs quite a bit less than some similar products, coming in at only sixty bucks. Due to the lower cost I wasn’t expecting anything overly special, so it came as a nice surprise the first time I gave it a whirl that it sounded amazing. The difference between listening to music though the jack on my pc and through the amp is astounding, suddenly the music just comes to life, and it sounds like you’re in the room as they’re recording it.
If you’re looking for a cheap (relatively) headphone amp, then I would definitely recommend the FiiO E10.