There is not way if you watch a movie decoded in 5.1 or above, like Finding Nemo or whatever, that it would sound better in Stereo Mode versus DD, DTS, PLiiX or whatever.
For starters, your taking the surrounds out of the picture, which is the whole basis of Home Theater. Yeah the center can be phantomed by the two mains, but will never give you the same purpose as decoded on the DVD's.
I think your setup is wrong somewhere, not sure. Sorry I can't help, I'll never get rid of my 7.1 setup, maybe upgrade someday.
ps: bring on 10.1
