Is the power transformer in the amp a toroidal?
It could be DC offset voltage on the mains.
This will cause the power transformer in the amp buzz loudly.
Causes of DC offset voltage on the line? Dimmers, low voltage lighting (lighting commonly used for upper kitchen cabinet lighting used to illuminate counter tops), cheap CFL lights, cheap 120V retrofit LED lights, multi wattage portable electric space heaters, hair dryers.
http://sound.westhost.com/articles/xfmr-dc.htm
http://www.diyaudio.com/forums/solid-state/2080-dc-filter.html
.
It could be DC offset voltage on the mains.
This will cause the power transformer in the amp buzz loudly.
Causes of DC offset voltage on the line? Dimmers, low voltage lighting (lighting commonly used for upper kitchen cabinet lighting used to illuminate counter tops), cheap CFL lights, cheap 120V retrofit LED lights, multi wattage portable electric space heaters, hair dryers.
http://sound.westhost.com/articles/xfmr-dc.htm
http://www.diyaudio.com/forums/solid-state/2080-dc-filter.html
.