Did anyone ever figure out exactly where the 4.1 track on the Blu-ray came from? It does seem odd to have a separate 4.1 if it's not actually based on the old 70mm mix.
I may have answered my own question. Did some digging on other forums, and there are claims that the 1995 THX LD's sound was actually based on a pre-release test mix that was in Sensurround (which had all the rough sound editing), and not the final six-track Dolby Stereo mix that was used for the film's theatrical release.
So it could be that the Blu-ray's 4.1 track is actually based on the latter, and is therefore a fairly accurate (or as accurate as possible) representation of the originally released 70mm six-track release. Does anyone know for sure if any of this is correct?