Jake

If you are just playing the source without any add'l receiver "processing", when you play a 5.1 source, you will not get any sound out of the rear speakers. When listening to a 6.1 source (DD EX, DTS-ES matrix/discrete), you will hear the same material in both rear speakers.

That said, most receivers will allow different modes to be applied. I don't bother with these, just play it as it was intended to be heard.

Alan