I am based in South Africa, and have recently bought the Pioneer HTZ-323 all-in-one HT system, which is equivalent to the DCS-323 in the UK. It has a passive subwoofer. When I switch on the unit, even without a disk in it, there is a hum from the subwoofer which is audible with the volume less than half way, and increases as the volume is increased. The humming stops if I disconnect the sub or mute the sound. I have checked the wiring, and tried repositioning the sub, but this has made no difference. I have read most of the threads on this forum related to this problem, but they mostly seem to relate to active subs. While I don't fully understand ground loops and isolation, I assume this would not apply to me because there is only one power source to the main amp. In any case, the sub is made of wood and plastic, so I don't think there is any metal to ground it to the amp. I am new to the HT arena, and my first experience is somewhat disappointing, so any suggestions or advise would be welcome, and highly appreciated.