Mysterious hum in system...

I hope that someone might be able to advise me. I have a buzz in my system that I cannot identify. First let me say that I had to make a compromise with the wife to get my 2 channel set up. I have an ARC LS3 preamp, Bryston 4bst, Naim CDX and Revel F30’s for my 2-channel set up. I also have a home theater setup that is a Pioneer Elite VSX-41 receiver and a Pioneer Elite DVD/CD player. I also have my High Definition cable TV box connected into the receiver via a digital coax cable.

The compromise is this: I had to have this system do ‘double duty’ as both 2 channel and home theater to keep the peace at home ( I am sure many of you can sympathize with this ). To accomplish this I have connected the Elite receiver’s ‘preamp out’ connection into the Bryston’s XLR inputs by way of Audioquest Diamondback RCA interconnects and a set of Cardas RCA/XLR adapters. When using the HT I set the switch at the back of the Bryston to the “XLR’ setting, otherwise for 2 channel listening I leave it set to the RCA setting.

The problem is that I have a hum in my system when I have it set to 2 channel RCA setting. The humming disappears as soon as the XLR's are taken out of the Bryston. I have put cheater plugs on the receiver & DVD player but it didn't help, nor does the ground loop switch on the back of the Bryston (it does make it buzz less though). When the XLR is disconnected my system is dead quiet though. Any advise short of the obvious (scrap the bloody theater system!)?