When attempting to track down a power / transformer hum elsewhere in my system, I discovered that the Centre speaker (a Monitor Audio Silver SLCR) was emitting its own, very good, approximation of a power hum. Swapping the centre for another speaker also trasnfers the hum. Further investigations reveled that the hum was also audible, though less so, through both my main speakers, (SM S8s). Is this normal? Has anyone any ideas for eliminating this hum? Checking the grounding? Cleaner mains, perhaps? A new sub-station . . . . ?