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.