No, Prologic IIx adds two addiional rear speakers aside from the regular two rear ambient speakers. The extra speakers are dispensible INMHO and probably have very little effect on the overall impact, although I use the rear speaker because I have them in my system. You will get virtually all the benefit from a good 5.1 ProLogic II system.
Do I need Pro Logic IIx?
I have a 5.1 system and am upgrading my Marantz SR7000 to separate components. I've already picked up an amplifier and am looking around for a second hand processor. Systems with Pro Logic IIx are newer and second hand units harder to find and more expensive than those with Pro Logic II. From what I've read, Pro Logic II supports five speakers and Pro Logic IIx supports seven speakers. As my speaker set-up is 5.1 and I'm not interested in 7.1, would I be losing anything if I pass on the Pro Logic IIx and go with Pro Logic II?