You wouldn't....you'd simply lose the rear info. TBH, if you were going away from 5.1 and just using a transport, then two channel is the only option that makes sense.

If you really wanted to use the 3 front SP's then you'd be better off keeping the 568 in the setup. At least you can do Trifield etc and also integrate the rear channel info into the front.

Rgds

Nick

Rgds,
Nick