At 60Hz it's clearly an earth loop. These can be frustrating to locate.
1. Plug a completely different system into the outlet you are using. Is the hum still there? Then it's the house supply.
2. Strip down your system as far as possible and see if the hum is still there. If so, substitute items in turn with outside items, including power cords, connecting leads. Eventually the hum will be absent and you will have found the offending item.