Well, since my original post I've had some breakthroughs on my little project. I've since gained the ability to separate the LFE channel from the special edition soundtrack directly, rather than having to record it through the receiver (thus avoiding digital-to-analog and analog-to-digital conversions). I also bit the Microsoft bullet and dug out a semi-old Windows laptop so that I could use the Aften AC3 encoder, which offers considerably more options than the limited ffmpegX implementation. Since the majority of synching had already been done on my first effort, all I really had to do was synch up to that version, touch up on places where I had been slightly off, and tweak the levels a little. I was able to skip the space-consuming multichannel wav step, since Aften could accept multiple mono inputs, and didn't have to create dummy files for unused channels. This time I encoded the file as a 2/0 + LFE AC3 at 448 kbps. There should really be a Mac version of Aften . . .
The following is a rundown of the various bass effects in the movie and what changes, if any, I made to them to fit better with the 1993 soundtrack.
Star Destroyer low rumble/Tantive hit/reactor shutdown: I boosted each of these effects by 3 db--the 93 soundtrack is so loud here that they weren't very noticeable otherwise.
Stormtroopers blast through the door/Vader enters: No changes necessary. The rumble just before the explosion is particularly forceful; Vader's appearance is ominous.
Escape pod launch: A subtle effect, not really necessary but neat.
Artoo falls over: Some people will probably disagree with me on this one. I left this absurdly loud sound effect in the movie, although I did reduce it by 1.5 db so it isn't quite so ridiculous. In the original soundtracks, Artoo hitting the ground sounds like a trash can falling over; in the special edition, it's somebody overturning a huge friggin' dumpster. :P It definitely veers into the territory of modern-day exaggerated bass, and my initial inclination was to remove it, but what the hell, I kinda like it.
Jawa sandcrawler: Two shots of the sandcrawler driving along. In the SE, the second shot was CGI'd, longer than the original, and much more bassy. I had to delete this sound effect since it didn't match up at all, but I replaced it with the same sound from the first shot, at slightly lower volume since the sandcrawler is farther away and the music is also quieter than in the first shot.
Star Destroyer approaches Death Star/Vader's Force grip: I think I achieved a "best of both worlds" effect here, since the special edition has deeper bass for the grip than the original, but the older soundtrack has a windlike upper bass that the SE seems to lack. Combining the two yielded good results.
Door slams in Leia's cell: A subtle effect, after Vader brings in the interrogator droid.
Millennium Falcon takeoff/jump to hyperspace: The Falcon's engines are reinforced by some seriously powerful LFE throughout the movie, beginning with the escape from Tatooine.
Alderaan explodes: I had a bit of trouble with this one, because it didn't quite blend the way other sounds did. In the SE, the bass, while loud, isn't nearly as loud as the explosion sounds coming from the main channels. It grows as the unfortunate CGI shockwave approaches the camera, and hangs on for a moment after the shot changes to Luke practicing his lightsaber. (The original explosion sound, of course, ends as soon as the shot changes.) The solution I eventually came up with was to increase the earlier part of the bass by 3 db and then to fade out the ending, so that it sounds more like the planet itself is causing it and not a (non-existant) shockwave, and it doesn't hang on quite as long or loudly afterwards. It's not perfect, but it's better than leaving it unchanged, I think, or not having it at all.
Millennium Falcon emerges from hyperspace/flies to the Death Star: Serious bass going on here. The ship's engine is very loud, especially on the flybys as it pursues the TIE fighter leading them to the Death Star. Quieter bass occurs as they are caught in the tractor beam and enter the hangar bay. But the best part of the sequence is just after they come out of hyperspace into the debris--an ultra low frequency sweep kind of effect that pulsates louder and deeper while slowing down. It's as if the whole ship were shaking horribly, and I love it. This sound is well beyond the capabilities of most main speakers; if you don't have a decent subwoofer, you're really missing out.
Stormtroopers blast into cell bay/Leia blasts garbage chute: Two loud explosions. I didn't change the volume any, but I did alter the timing by shifting them both forward about a quarter of a second. In the SE, they have the curious quality of occurring just after the main channels, which works fine there, but when combined with the different explosion sounds used in the '93 mix, sounded wrong. Changing the timing so they occur simultaneously with the mains was much more satisfying.
Garbage chute: Various quiet sounds reinforcing the walls closing in, etc.
Chasm door slams: This doesn't really stand out if you're not listening for it, but it's neat.
Escape from the Death Star: Bass in the Falcon's engine, the ship shaking just before the fight, and the TIE fighter explosions. Cool stuff.
Falcon approaches Yavin: More of that powerful engine stuff, but less loud.
Death Star approaches Yavin: Deep and ominous. First shot seems louder than second.
Hangar bay/X-wing liftoff: There's a really quiet bit, I think it's just after Han says "I know what I'm doing", and a louder bit as Luke's X-wing rises into the air.
Battle of Yavin: Lots of bass here, obviously. The CGI shot of X-wings flying towards the Death Star had LFE that grew louder as they came towards the camera and faded as they flew away, which didn't match the model shots. I reduced it by 10 db, making it a subtler sound that blended much better. Explosions sound really cool, some louder than others. The Y-wings descent into the trench has bass; closeup shots of the massive turbolasers firing have reinforcement as well. Only a couple things I needed to change at the end: for the Falcon destroying Vader's first wingman, the LFE synched up correctly, but for some reason the effect was too long and started way before the actual explosion (the ending is on time). Fading in so that only the explosion part was loud seemed to solve that. The Falcon flying away from the trench was too loud, due to the CGI replacement swerving about (hey, look how cool our new effects are!), so I reduced it by 6 db. As for the Death Star explosion itself, it wasn't nearly loud enough, but the absurd shockwave was much too loud, again. I increased the beginning of this by 6 db--I mean, come on, don't you think the friggin' Death Star exploding should be the loudest bass in the movie?! Where's the sense of proportion?--and reduced the shockwave part by 3 db, so that it comes across as a little aftershock rather than a huge thing.
Whew! That's a lot of bass. Of course none of it's really necessary; the original soundtracks don't need to be changed. They are immortal, classic. I make no claims towards authenticity; this isn't really an attempt to recreate the 70mm soundmix or anything like that. I just think it sounds cool to combine the bass from the special editions with the oldschool sound mix that has the best frequency and dynamic range. The bass is all encoded in its own LFE track, of course, I didn't make any changes to the 1993 mix itself. It is still straight 2-channel, with ProLogic encoding. Playing the AC3 file with ProLogic 2 seems to discard the bass channel for some reason, but DTS Neo:6 retains it, thankfully, although the placement of elements is probably different.
If anybody is interested, I can of course share this effort. The good news is that since so much of the LFE channel is silence, lossless compression reduces the file size to less than 10 megabytes, making uploading and downloading a piece of cake! I'm currently working on doing the same thing for Empire, and eventually for Jedi. I don't have the PCM tracks for those two movies, though. I would certainly appreciate it anyone could upload those or inform me how else to get them (no newsgroups).
Long live the original trilogy!