I recently bought the AKG K-240 and they are not as loud as I'd desire. I have a new synthesizer on the way, the Roland SH-201, and I'd like to be able to use these headphones with it. What would be a decent headphone amplifier that'd work well with my synth and headphones for around $30-50? I apologize if this question is a bit redundant, I don't know very much about headphone amplifiers and the faq seemed a bit bare in that regard.
Edit: Also, I am willing to stretch my budget as necessary to comfortably obtain quality, louder, volume. Preferably something that doesn't work off of batteries.