The only time I ever experienced this was way back when I had my subwoofer plugged into a different circuit. Also be sure that any signal carrying cable (satellite, antenna, cable) also goes through the panamax to get properly grounded.

The Panamax doesn't know one component from the other when you plug them in, so if none of the above applies you might want to try unhooking components from your 990 one at a time to see what stops it.