The "down" in downmixing refers to the fact that you're reducing the number of channels. The opposite of upmixing (e.g., from 5 channels to 7 channels). If you're going from 3 channels to 3 channnels, then that is not a downmix.

As for what PLII does in the front channels vs what the 'no centre' setting does, it's quite different:

PLII does matrix decoding and logic steering. First, correlated mono information is copied and sent to the centre output. Next, anti-phase cancellation signals are generated and sent to the three outputs. This surpresses L/R stereo info in the centre channel and surpresses centre info in the left & right channels. As part of the logic steering process, the gain of the three channels are ridden based on where sound is dominant in the front soundstage. This aids the cancellations signals in enhancing the sense of channel separation.

Compare that process to setting the speaker configuration to 'no centre', where discrete centre info is sent to the left & right speakers and the centre output is turned off. To be blunt: this is the electronic equivalent of a simple Y-splitter.

I don't see how you can consider these two processes to be even remotely similar. Making the downmix function variable would require actual mixing capability, which is why I suggested you get an inexpensive analogue (pro-audio or DJ) mixer. Wait a second: you're only using two speakers? Rene, have you been guessing about some of the stuff you're claiming or have you actually tested those things out (as I did last night)? What extraction are you talking about? With two front speakers, PLII is not extracting any centre content. If you're using two speakers total (no surrounds), then PLII is not doing any processing whatsoever. Two channels, two speakers; what's there to process? You simply route each channel to its respective speaker.