(in other words they make up the channel content for those height speakers). Dolby Pro-Logic IIz is really just a fancy new version of Dolby Pro Logic II, DTS Neo 6, and DPLIIx. All of them are making up information on the fly for 5.1 and 7.1 setups. Sometimes they produce interesting results. Most of the time I find them to be a waste of time.
However, real, discreet 7.1 content is a different story. Some Dolby TrueHD and DTS-HD MA soundtracks on Blu-ray discs (about 1 out of every 7 or 8 discs in my experience) have 7.1 audio. And some DVD's that have DD-EX have those 8 discreet channels too. There is enough content out there for these systems to warrant a 7.1 system in my opinion. And unlike with the Pro Logic system, its all presented exactly the way the filmmaker and audio engineer intended.
If you have the room to set up a 7.1 system I say go for it. Skip the PLII-z nonsense and just focus on getting a quality 7.1 system. Like many others, I'm stuck with 5.1 because my living room is just too small and my couch is against a wall. So there's no room for those rear speakers.
I am planning to change my 10 yr old 5.1 system. Obviously, I want to upgrade to the latest which is 9.1 sys. But the dealer advised against it insisting that 5.1 is good enough. He claimed that there is hardly any disc that is recorded in 7.1 or 9.1 format. Any of you guys have experience with this ? I am planning to buy denon 4810 that has pro logic llz.Also , how important are the side speakers versus the height speakers?