Fran, there may be a better way for large venues. Actually, Bose recommends that each member of a band have a PAS behind him so that audience members can localize the sounds and experience some spread. I have attended a rock concern where this approach was used with PASs, and it works. This suggests that using multi-mono instead of stereo might provide spread even for large venues. For an arranger player, this might mean panning all instruments far right or far left. (If the keyboard has additional outputs, each can drive a different speaker in mono. Expensive, but should work.) As long as there is no common signal between speakers, there should be no phase cancellation. Also, it might be satisfactory to use weak (e.g., internal) stereo speakers with a PAS so that at least some people get a stereo effect.

Basically, if you are an arranger player contemplating the purchase of a PAS for mono use, just realize there is a significant compromise in spatiality involved.