First of all: if you've ascertained that the ac isn't too high then the manufacturer should solve this problem at his expense. If you have access to a variac you can reduce the ac output in order to include or exclude that as a potential source. A too-loud hum is still too loud at half volume(it shouldn't be audible except perhaps in the very immediate vicinity). Since transformer noise is usually mechanical,the fix should be simple. The question of concern is "who pays for it?"
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?