Let me jump in real quick. The Dolby PLIIx processor does allow you to take any 2 channel or higher encoded material and play it in 7.1. The results vary in the rear center channels.
For example, in DPLIIx, the two rear speakers will play the 6th channel info when a DTS 6.1 encoded DVD is played. It will also play the 6th channel from a DD-EX encoded disc. In both instances, the rear two channels will be mono.
However, the DLPIIx will "create," a 6th and 7th channel in the erar using steering technology based on cues from the left and right surround channel signals. The information will not really be any different than what is contained in the surround channels (that is, you're not losing info if you only have a 5.1 set-up), but the 6th and 7th channel will help to spread the side surrounds into the rear.
I think that the benefit is minimal, especially given the dispersion characteristics of the QS series. But, in a larger room, with some extra room in the rear, the 6th and 7th channel can really help to "open up" the surround field, and make the experience more enveloping.
