I bought an Antique sound lab tubes headphone amp on audiogon, used, around $200, i love it, another maybe more expensive choice, go with a benchmark dac which has headphone jacks, you will get a massive improvement in digital playback and a headphone amp

good luck

Kurt

Grado actually makes a headphone amp that is powered by a single 9v battery that might be perfect. I listened to it a number of times with the SR80 headphones and it is much better than any headphone output that's built into a component that I have tried. Perhaps that should be on your list of products to audition. It's a bit above your stated budget, but worth a listen. Project also makes a nice affordable headphone amp.

It might be a little overwhelming at first but the Head-fi forum has way too much information about this kind of stuff and way too many people willing to give advice.

Head-fi forum - Amps

I ran my Grado RS-2's with the Grado RA-1 (9V powered) for a long time, and I liked them. Maybe look for a used one. But I now have a small collection of tube and SS amps. All unfortunately to my pocket book are all over your current price point.

I have one that works and sounds pretty good:

http://www.pha.inecnet.cz/macura/buffer_en.html

Looks like this:

http://web.telecom.cz/macura/head_photo.html