Sometimes I winder if I should try to do people's troubleshooting for them. But then it is hard to earn at first.
Looking at the print, you see where the volume control is. since the buzz does not change with the volume you look downstream for the problem. It could have been the main caps but they would have to be about dead open circuit as amps are designed to tolerate fluctuations in that supply, because when you load it and crank it, it is going to fluctuate.
There is a tone amp after the volume, some manufacturers call it a flat amp I think, but it is before the PRE OUT/MAIN IN jacks. If the buzz goes away when you pull those jumpers the most likely suspects are C453 on the tone board and C617, 618, 605, 606 on the regulator board. Also,looking at the design I would say that more than one of these has to be bad.
For the money, if you can't test them, just throw all of them in and 99 % chance it will be fixed.
