We have the schematic, and I will be taking a look at the amp and Thermionic's suggestions this week.

My brother followed up, he did some basic trouble shooting. The amp in question is a Peavey Heritage. It has a solid state preamp section and tube power section. He ran a line from the pre-out to the pre in on another amp (a Carvin). And the preamp section is A-OK. He also ran the other amp into the speakers on his Heritage, and the speakers are OK. So, that did narrow the problem to the power amp.

He said the amp makes a "ring modulator" type noise. Thermionic had some good suggestion on capacitors in the circuit that could cause that problem if shorted out.

So, check in the guitar amp section as we troubleshoot this and solve the problem.

Thanks.