Preamp Switching to Mono

Maybe I am doing something wrong but this is my problem.
I have a classic Audio Research LS3. When I just use one pair of the outputs the preamp puts out stereo, all is good. But if I use the second (other) pair of outputs, (in addition to the main one) to feed a electronic crossover for the subwoofer, the preamp for some reason is putting out mono on that original pair of outputs. (I want to use that second pair of main outputs instead of the tape outputs as I need the gain for the subwoofer.)
Does anyone have an idea of why the use of one set of outputs would have an effect on another? Thanks for any insight.
Are you using Y cables on the second output? You need to make sure and use stereo outputs into the crossover as it can sum the main outputs of your preamp if they are not buffered. In addition, don't use the tape outs because they will not be controlled by the volume control.
Thank you Rwwear,
Yes I was using a "y" running the second pair into a Behringer electronic crossover that has a separate subwoofer output but only the one input. Can you think of a way around my dilemna short of selling the Behringer and springing for a real subwoofer crossover/amp? I usually only use the subwoofer for movies so I was also thinking of using my HK receiver (which I also use for the surround speakers), and feeding that sub output either directly into the power amp for the sub or passing it through the Behrenger first if it gives me more control over the sound level.
Thanks for the info!
Yes, just run one cable either left or right should be fine. Stereo would be preferable but bass is usually summed at low frequencies anyway as long as the xover point is low enough.