Kevin, How do you figure? PLIIx and CES work quite differently.

PLIIx will take the logic steering that PLII already does and expands it into the 2 surround-back channels. This means if a certain sound is meant to image in the left rear channel, that sound will also be actively supressed in the left side channel for better directionality. You already hear this sort of cancellation with PLII on the front 3 channels, where the centre channel content is supressed in the front left & right channels, keeping them free of dialogue.

Compare that to the simple electrical summing that CES does. Common sounds in the left & right surround channels are simply sent to the surround-back channel, without any active steering or cancellation. Unfortunately, that means any mono sounds in the surround channels will be heard from the sides and rears simultaneously, rather than only the rears.

Add to that the fact that PLIIx will generate stereo rears while CES sums to a mono rear, and the two modes shouldn't sound "very similar".

Best,
Sanjay