If your amp has ground plug it means it was designed to have it. DO NOT use cheater plug or cut ground wire - it is dangerous.
It might be ground loop or something injecting noise, but it can also be filtering caps in power supply getting old (dry) or something wrong with tubes. Your amp has tube rectifiers - perhaps you should have them checked?
It might be ground loop or something injecting noise, but it can also be filtering caps in power supply getting old (dry) or something wrong with tubes. Your amp has tube rectifiers - perhaps you should have them checked?