Tripper, It's common knowledge... always put your Subwoofer on top of the Preamp to prevent standing waves in your bathroom.
But if you want the actual answer. I connected the negative lead to the preamp chassis ground. Not a good idea, but that is the advice I got from Sumiko, the Rel distributor on the west coast, to make a balanced connection to a stereo amp. Honest.. I think... The Great thing about Pass Labs is they fixed a competitors preamp that I screwed up! Ever heard of a company doing that?