Pro Logic IIx does everything that Pro Logic II does, just with the added ability to extend itself out to six or seven channels. In the case of a 5.1 setup, you can still use PLIIx with two-channel sources. It's only those 5.1 sources which are already using all of the available channels of your system without any additional matrix processing which you will be unable to select a "DD+PLIIx" or "DTS+PLIIx" mode.