If you're basing your expectation of Prologic IIx and IIz on what you heard from the original 4 channel Prologic; you're going to be in for a pleasant surprise.
I agree that the old Dolby Surround sound (a mono surround extraction), and Prologic (added a center) left much to be desired. But these days my receiver is always in Prologic IIx mode. I only switch between "Movie" and "Music" depending on the media.
The problem is DVDs are only 5.1 and even most Blu-rays don't offer 7.1 (there's no format with a discrete 9.1). With my 7.1 setup, when playing a 5.1 movie nothing gets sent to the rear surround speakers when in native Dolby Digital or DTS. Because my surround are completely to the side, pans give the illusion of moving over head instead of behind me. But enabling Prologic IIx has the the processor analyze the surround channel and steer the audio around to the rear surround channels also, for a very natural sounding effect.
For stereo tracks Prologic IIx does ambient extraction, finding the natural cues in the mix to expand the sound field. For older matrix surround (Dolby Surround/Prologic) movies it does a better job than the encoder for which they were intended. In music mode the effect on acoustic music is wonderful, and electronic music becomes even more unreal.
I'm a fan of DTS's encoding technology, but their Neo:6 does nothing for me. Which is another reason I prefer sending LPCM to my receiver it doesn't stick me with Neo mode just because it detects a DTS track. Dolby Labs has really got the science of processing sound down.
As soon as there's a pre/pro which has the feature set I want and 9.1 Prologic IIz, I'll be right in line to scoop up two more QS8s to put on my front wall.
