Another option is to just replace the capacitors, while this may not fix all the amps it did fix two of mine so far, which were doing the same thing as yours. If you can do basic soldering (or know someone who can) it is worth a shot IMO. You can get the caps from radioshack for about $6. There are instructions floating around the forums, and it is pretty easy to do. You can remove the speaker grill and housing and unplug the amp to find which one is the problem, if you are having trouble finding which speaker it is.
Good luck.