Hi, I have an annoying mains type hum from my Sony AV amp that remains constant regardless of the volume setting. It can be heard equally through all 4 speakers and the sub (I dont have a center). I have tried the following without success: - Re-routing signal cables away from mains cables. - Unplugging the coaxial digital input and switching to an optical digital input. - Unplugging the subwoofer LFE phono and turning off the subwoofer. - Unplugging the front L/R pre out phono's. - Attaching an earth lead from the AV amps ground post to the earth pin of a plug. Does anyone think I have a faulty AV amp? I only use the AV amp to power my rears. The fronts pre-out to my main 2ch hi-fi amp. Thanks.