Wow, thank y’all so much for the responses.
Star Wars in 1977 had three different soundtracks (6-channel, stereo, and mono), which had different content (although the 6-channel and stereo were very, very similar).
I’ve seen some comparisons of the stereo and mono before. Do you know exactly what the differences between the 6-channel and the stereo were?
Empire had three original mixes, the 35mm stereo, the 16mm mono, and the 70mm 6-channel. The 70mm version was a different video cut, and we don’t have a good preservation of that (there’s an 8mm digest which has bits of it, if you want your mind blown).
Do you know which versions (official home release or fan preservation) use each TESB mix? I’m inclined to believe the Grindhouse would use the 35mm stereo, insofar as it also uses the 35mm video. Assuming the 70mm 6-channel audio is lost along with the video, is there a good place to find the 16mm mono track?