If you have a 5.1 setup, PLII/PLIIx decoding would only be applicable for sources that are less than 5.1 (such as Dolby Digital 2.0 or something really unusual like Dolby 4.0). Weird fact about PLIIx: it is often marketed as a 7.1 extension of PLII, but it really is more of an update to PLII that adds the ability to go up to 7.1 - PLIIx applied to a stereo source on a system with fewer than 7.1 or 6.1 speakers (such as a common 5.1 system or even a 4.1 system with no center speaker) will honor the number of speakers available in the system and output accordingly.