Anyways, I think using the Dolby Pro Logic IIx 6.1 decoder will yield better results even for mono surrounds than Dolby Pro Logic II 5.1 or Dolby Pro Logic 4.0.

Not necessarily. Decoders/upscalers decode to whatever output config you select.

The upscalers have NO KNOWLEDGE of the config of the source material, all they get is 2ch something. And they WILL upscale to any number of speakers you select.

DPL2x is 6.1 OR 7.1 (Again, whatever you select). DPL2x will (generally) NOT increase the discreteness of your upscale by a large amount, because there is only so much info you can store in (matrixed) 2ch (in fact, the most sortove discrete ch you can store in 2ch is 3ch with the best DPL2 decoding, you can only get more with extremely controlled synthetic conditions.). Using professional 5.1>7.1 upmixers, what’s in the back 4 is basically the same as the previous 2.

You can>: decode to 5.1 (DPL2) and mono the rears. OR decode to 4ch (DPL1) and stereoize the back.