When I switch the receiver to Dolby Pro-Logic mode, I get a 60cycle hum from all 5 speakers....when I switch to Stereo or Direct (stereo) modes, the hum goes completely away.... I am baffled! anyone know how I might track this down? It appeared after our cable install today, so at first I suspected a ground loop in the cable system itself....but I can't figure out why it would only affect the sound in pro-logic mode, and not stereo. EEK!