Recently had my plasma installed with various bits and bobs attached and every time I turn off the amp to go to bed the Sub starts humming and I have to turn down the base level on the front of the sub. Can't hear it at all when the amp's on.
Mine has been in for repair since March now mate... apparently SuperFi can't find a problem but I bet you the buzz is just as irritating to you as it is to me! I bought the system 31 December, it broke after 10 weeks and is 'being looked at' ever since.
What makes mine more irritating is that I've had the sub hidden in an open brick fireplace and I have to get in all sorts of positions to reach the bass knob on the sub. I'm having it looked at next week.