Leo,
What you're suggesting is a waste of time and money. The whole idea behind the Bose is to save space and spread sound. You don;t have to hook up 2 other speakers ... you need to get a hi end sound that sums well to mono, or fix the sound in the kb you have.
This whole discussion of why the Yamaha piano sounds so bad in mono has NOTHING to do with the Bose. The sample is just weak, that's all. It relies on stereo effects to give it charachter. I tweaked the sound on the 3k Live Grand and it works very well in mono. You need to edit it while you listen through the Bose.

Using a system like you described is like selling the Bose short. If you MUST hear the sound in stereo ..... just don;t get teh PAS. It's pretty simple, really. But, if you have an open mind, and an open ear ...... really give this baby a test because it can save your back, simplify your life, increase your effectivness and improve your visual. Whew! What a product.

Hey !
Bose Marketing/endorsements .... is my check in the mail yet?