Hum!

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? 

Try plugging all equipment into the same outlet. Different outlets can introduce ground loops where a small voltage difference exists between the different grounds causing current to flow between grounds- a ground loop.

Thank you for the information, I will make some tests.