I was at an estate sale a couple of days ago, and sitting the a table was a pristine pair of AKG K240s headphones. I snagged them for $5. I attempted to use them on my MacBook air and my iPhone, but the volume output is super low. I then amplified them with my Fiio E7 portable amp. The volume is much better, but even with the amp maxed out it is still a bit low. Any of you have a good amp recommendation for the K240s headphones? Portable and desktop versions are needed, and $250 is about the max I can spend on either.