That is strange that's it's happening with both amps. I'd be getting those amps to a good repair man. It's got to be something in the effects loop or the pre-amp section (probably the effects loop like pstring said). If you have a pre-amp, such as a POD, try plugging into the pre-amp, then plug the pre-amp into the return of your effects loop. Nothing will be plugged into the send of the effects loop. This will bypass most amp's built in pre-amp (if the eq controls don't work when hooked up this way, the amp's pre-amp is being bypassed). Just a way to check to see if the problem is in the pre-amp section.