I can't speak about the Yamaha stereo situation but, it is not at all necessary for me to tie up both preset channels with my KN7000.I find it much more advantagious to use one for the kb and the other for my mic with the proper preset, as Bose recommends.

As far as the number of insruments being fed into the PAS is conscerned, it is not a matter of overloading the system. Bose says it creates the "cocktail effect". This is where the audience can't discern where the sound is originating from. Ideally,every member has his own unit,but for all practable purposes this is not the norm. I am sure there a marketing consideration in play. Just be careful with the step gain when using a mixer. The trim should be pretty well down to nothing on Channels one or two.

Bernie