Like Pro Logic, PLII derives the center and rear channel information from the front L and R channels. DD5.1, which has five discrete channels plus a subwoofer channel, should give better results if a disc is 5.1 encoded. However, PLII is an improvement over Pro Logic for older, "Dolby Surround" encoded soundtracks.
Unless you have a large collection of laserdiscs of VHS tapes, PLII isn't reason enough to consider an upgrade, IMHO.
Unless you have a large collection of laserdiscs of VHS tapes, PLII isn't reason enough to consider an upgrade, IMHO.