Actually, Dolby Pro Logic IIx is 6.1 surround sound.
And it may be better than using plain Dolby Pro Logic 4.0 or Dolby Pro Logic II 5.1 as you said.
I think the Despecialized Editions should be in DTS-HD Master Audio 6.1 Discrete in the next versions.
That would be really awesome.
I didn’t care much for 6.1 on the Star Wars: The Complete Saga Blu-rays but I absolutely loved the 6.1 surround sound effects on The Lord of the Rings Trilogy and especially on The Studio Ghibli Collection.
Just turn on the Dolby Digital EX flag for the DVD-5 and AVCHD DVD-9 versions while the MKV version is a discrete DTS-HD Master Audio 6.1 track.
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.
You have to have the audio source to warrant it. The Pro Logic decoders can only decode what was encoded in the audio in the first place, they do not create the channels, they just decode them. So when you run the original 77 Star Wars Stereo mix through a 5.1 decoder, you only get 4 channels. Run it through a 6.1 or 7.1 decoder and you still only get 4 channels. But the decoder knows what to do with those channels.