[No-One]

I've got a Pioneer VSX-808RDS amplifier/receiver and a Yamaha YST-SW90 Sub woofer. I connected the Sub to the Subwoofer pre-out for dvd and it worked fine. BUt when I try to play music with the sub enabled nothing heppened. I thought that the amp probably doesn't output via the pre-out when not in surround modes. So I connected the standard inputs on the sub, to speaker set B on the amp.
With just speaker set A enabled, I get sound through the main speakers and a very slight sound from the sub. (which I still don't understand as set B shouldn't be outputting anything. With just set B enabled I get sound through the sub and nothing from the speakers. But, when I enable both sets of speakers, I get nothing from the main speakers and sound from the sub. After about a second or 2 I get a loud constand hum from the sub, even with the volume turned right down. What's going on?

Also, if this is an imcompatibility between the set b speaker out, and the sub, why don't I get sound through set a with both enabled? I tried disconnecting the set B cables and turned on both sets, still nothing from the main speakers.

Paul.