I wouldn't even pop the cover off of any equipment that's still in-warranty; that could work against you. Transformer laminations are probably buzzing so it needs to be replaced. Line voltage variations shouldn't matter, even +/- 10% in a proper design. Vantage's suggestions above have merit, but I would experiment along those lines only if the unit was out of warranty. Once you go in there fooling around, the manuf. has an easy way out. Take the amp to a friend's house & power it up there for a comparison. No need to connect inputs or speakers to try that. It will probably still hum no matter where you plug in. If it doesn't, only then does your AC line appear suspect. Hey it might just be some loose mounting hardware, but get the manuf's OK - IN WRITING - before you go inside.
Why is amp toroid humming?
My 6 week old Belles 150A Hot Rod amp has started to emit a humming noise. The volume is such that it can be heard from 4-5 feet away in a dead quiet room. David Belles, the amp's designer, is being helpful and suggests that line voltage over 124v. could excite the toroid, as could line interference from rheostats, etc. However, the amp ran silent for the first six weeks and nothing has changed in the electrical system to my knowledge. Time of day/night makes no difference in presence or volume of hum. Disconnecting speaker cables or inputs makes no difference. Nor does changing power supply to a different house circuit. Anyone had experience with a problem like this?