I have sucessfully wired a simple cheap stereo potentiometer, obtained from Radio Shack, to the line-level outputs of stereos before. It should be no problem, a very simple operation, once you know how to solder it.

I actually had to have some help on the empeg BBS regarding which wires get soldered to which pins, because I was confused about how that sort of thing worked. I'll see if I can dig up the thread.

If you don't feel like getting out the soldering gun, I'm sure someone sells that sort of thing as a pre-made box.

Remember it has to go on the line-level side of things, not the speaker-level wires.