Scott,
You can't get the true benefit of the 5.1 because your kb transmits in stereo only, but you can run it through a mixer that has separate outs, maybe.
Try this:
Main L & R to front speakers
Sub 1 & Sub 2 to rear speakers
(send at least the drums to these outputs, and maybe all or some of the acc tracks)
Use the loop out to feed the subwoofer ( or the HP jack.

This won't give you true 5.1 but the separation of the drums will add a fullness and added spacial effect to the mix.
If the loop out isn;t strong enough for the sub, then try the HP jack. A mono cable is fine .... the sub is mono anyway.