Could be reversed wiring in the electrical outlet. Buy a 3 pronged polarity tester at your hardware, Home Depot or similar. It has lights that indicate what kind of connection problems could be in the outlet. It's a good idea to have one anyway...are you sure every electrical outlet in your home is wired correctly?
A missing ground connection or a reversed hot/neutral connection could cause the hum. Also lights or appliances in that room or on that circuit that the amp is plugged into could also be a source of interference.