A new development in tone suck: FM interference. I'm actually picking up a radio station through my guitar amp. I'm thinking dirty power is to blame.
Having the amp overhauled and especially the grounding checked could be benificial, as would power conditioning. Any peice of wire can act as an antenae picking up all sorts of impurities around 100 MHz, but it shouldn't be too tough to filter out all but the 60Hz sine wave for power. Costs of typical power conditioners belie its simplicity. But this one seems reasonable.