You might check with your preamp manufacterer first. I did the same thing and once I plugged in the "Y"-adapter to my extra set of outputs, it bridged both channels, (and both sets of outputs) to mono.
If the extra set of outputs are just paralled off the first set, this will happen.
I had to go with an outboard subwoofer crossover that took the stereo signal from the preamp, then converted it mono out to the sub.
If the extra set of outputs are just paralled off the first set, this will happen.
I had to go with an outboard subwoofer crossover that took the stereo signal from the preamp, then converted it mono out to the sub.