I run my receiver in PLIIx all the time. With true 7.1 content it does nothing, with 5.1 it just steers the surround channels around the rears as needed. With 2.0 content it dematrixes the out of phase material to the surrounds and rears, while moving the mono content to the center.

There should be no problem with running PLII all the time, when fed 5.1 content it'll do nothing.

Now, if for some reason you're limited to 2.0 content from your player (how are things connected?) then there will be a difference in the surround processing modes.

So the question becomes, what player do you have, and how is it connected to the receiver?
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Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris