Without hunting around for something that could sum signals (may exist, just don't know of a place for it right off), there's always the possibility of compromising and just using a daisy chain from one of the channels (left or right) to feed a signal to the transducer's amp channel. The LFE gets split equally between the two, so you'd get it - you'd just get half of the crossed-over bass (either the right half or the left half).