AFAIK, CMSS == Prologic.
The rear channel is a blending of the left/right rear audio. When things are behind you, you can hear them back there, but you can't seperate the left and right.
Just buy the adaptor, plug it in, and see for yourself. The adaptor costs about $5 CAD. It's what provides the audio for my GameCube, and I am quite satisfied with it. Given that I have a Soundblaster Live!, I have full 4.1 audio.