My piece was bought at a discount because someone returned it,after the first switching on I wasn't surprised, the hum was really extreme. But as soon as you plug the guitar and play the strings you stop noticing it, the sound is just rounding... After a few experiments I found out that the hum (50 Hz from the mains) is mainly amplified by the position of the high voltage wires inside the amplifier, in my case it helped to fix them as close as possible to the board with potentiometers.But beware,I don't recommend touching anything unless you know what you are doing,it runs over 400V,even after switching it off it can kill you!.Another thing is the Tremomla circuit in the EU designed pieces and a few other things,but there is no room for that here,goooogle. So hopefully this will help someone.Good luck.