I don't thnk using your keyboard will damage your speakers??? Just don't overdrive them. What will damage speakers is the amp not having enough power thus causing heavy distortion (clipping) at high output levels...That will blow speakers.

You typically only have 2 main and 2 aux outputs on a board... no sub woofer specifically (except Tyros). so you certainly are not going to get 5.1, 6.1 or 7.1

Lee