Hey Tony,
On my Korg PA 80 and my Yamaha PSR 750 there are left and right stereo outputs. On both keyboards the Left is labelled as L/mono. Does that mean that the keyboard already routes both channels to a combined mono output or would I be wise to plug a Y connector in to both outputs before plugging into the Bose?

As far as sending out a stereo sound image throughout the room you would never hear the stereo image like you do on headphones even if you were in the sweetest of "sweet spots"

All I know is what I hear from slightly behind and to one side (without feedback) from the Bose is the best sound I have heard from my setup. My friend checked it out in a church hall where I was playing. He told me the sound he heard from anywhere in the room was the same as what he heard when he stood beside the speakers. Combined with the fact that I was hearing myself better than ever before that info was enough to convince me to turn my rental into a purchase.

God Bless,
Don