Mark,

I have to say the plugs have been switched round so many times I may have got them mixed up. It was definately the rear speakers, although I hardly ever use the fader/balance control, so it could have been the front pre outs feeding the rear channels on the amp. It's all still repeatable, there's whine on one pair and silence on the others. I'm not in the mood to remove the sled and have a look as it's far too cold outside. I'll probably have another look when it gets a bit warmer.