could be all sorts of things--cable routing, inadequate cable shielding, improperly grounding and ground loops from antennas or wall outlets, as well as bad caps or relays. since i understand you still get noise when you remove all the sources and when you swap speakers, it's probably not a cabling issue ( i would try using different cables to eliminate), so before start soldering, i'd try a couple of things:
1. plug the amp into different outlets and see if the noise recurs--you can also try using a ferrite choke or hum eliminator adapter.
2. unplug the amp, let it sit for a couple of days, then try screwing down the transformer really tight (there's a philips screw right in the center)--it could conceivably be some mechanical vibration--be careful.
