This is a ground loop...
Many times this is caused by cable TV being hooked to the theater system. When you unhook the cable TV, the buzz will go away.
Install a Cable TV ground isolator transformer between the cable TV and your audio equipment.
Install 3 to 2 prong adapters on the grounded power plugs of the equipment. These adapters can be bought at the hardware store.
After "lifting" the grounds, the buzz will go away.