I read somewhere that certain Onkyo's could not be configured to send bass to the front speakers, so they cut out the bottom end if you didn't have a sub-woofer. Not sure which Onks it was.
Geez, buying a receiver these days is like buying a computer. As soon as you get it, there's a new feature set out and its outdated. I couldn't wait, b/c old receiver dies a couple of weeks ago and we were going stir crazy w/o music. New Denon sounds sweet. 5.1 is clearly an established format for movies, and many dvd's include it. I don't think there are many 6.1 movies out yet.
What happens if you play a 6.1 movie over a 5.1 receiver, is there built in down-scaling?
Ten, there is no downconverting going on. In the case of EX and Matrixed ES, all of the material is present in the left and right surrounds. DTS ES discrete is a different story, but since it is backward compatible with ES matrix, the info is still present.
Just make sure if you do not have a rear center surround(s) hooked up that you do not have your receiver set for 6.1 playback. This would cause the center surround info to be pulled out of your side surrounds which could have an undesireable effect.