I own a Bryston 4B-ST and had the same problem. How did I fix it you ask? Take a wall plug adapter and place it on the end of the three prong plug at the one end of your amp power cord. You can find these adapters at any hardware store (Home Depot, etc.). Your amp power cord has three prongs, the third prong being the ground prong. This hum you speak of may be a gound loop in your system. Before going out and spending big money on new cable, try spending 30 cents on an adapter. It should work.
Good luck and happy listening.
Good luck and happy listening.