Okay I think I found something of consequence. Okay you just want 2 channel stereo out of your 5.1 system. By this you mean you want the left channel to go to the left front and left rear, the right channel to got the the right front and right rear, and the center and sub to be whatever they come out as...

Some how I came across this basic little adapter from logitech

please google

970150-0403

It's cheap and just might do what you want it to.