Good advice Firehead, expect that there is a problem with the Yamaha arrangers in particular in that they just don't sound nearly as good in mono as stereo. I suppose, as many have said, that the problems is in the panning of effects or layers, but it does exist. If it weren't for that, mono for live performance would most likely be the best option, since it would be easier to cover the room with the full sound.
DonM