The debate in this thread is very interesting.
I think at this point it's pretty much an established fact that Garcia did NOT use any piezo saddles or pups in any way.
There are those few songs that Acoustic sound is very prominent to where it almost sounds like jug band acoustic, & I still think there's some debate as to whether he was using an acoustic simulator in his Roland midi unit, but I don't believe he was.
I believe what brad said earlier in this thread, like 3 yrs ago lol, about it going direct into a full range speaker is what caused that acoustic tone.
The reason I think this is because I've used a full range PA speaker, a JBL PA speaker for my bands PA that has a 15" & a tweeter. There was a minute that I didn't have my JBL cab & I was experimenting using PA speakers with interesting results. I actually liked it with my Mac for the most part but def wasn't as good as a regular guitar speaker.
Going through a full range speaker def gives you somewhat of a acoustic like tone, the middle pup tamed this a lot & you can get a Garcia tone with it but it sounds more like 93-95 jer when he was sounding good, there was still a lot of good tones that he got.
When you switch to the bridge and bridge/middle mixed positions it took on a very distinct acoustic tone, very much like the acoustic tones Garcia got in those last couple years. Bridge pup on single coil definitely sounded acoustic like. I played my friends PRS with real piezo pups immediately after & to my ear it sounded almost identical.
For my money I would bet that it was all from jer going direct To board & then full range speaker which caused that effect, just like brad said earlier. I believe he got the really acoustic tone by going to the bridge pup single coil. The bridge pup with a dimarzio split sounds very acoustic through full range speakers.
Personally I feel like Garcia was going with a new direction in his tone quest & never quite acheived what he was shooting for before he died. I think if he was given a chance to get healthy he wouldn't have ever gone back to his old twin tone, I think he was going somewhere cool.
If you listen to his tone through the years it was on a slow & mostly constant clean track, it was always getting cleaner. I think he was shooting for a really clean, almost acoustic like tone. I personally think if he was healthy in those last few years we would be talking different about that tone. There was still some incredible shows where he got it right & his tone sounded great. I think his fingers just weren't working as well as they should've. I listened to the final soldiers field show & those last few songs were really bad, it's sad. His solo in black muddy river, the one they overdubbed in a solo from another night is just sad. He never gets it going at all & you can just hear that he knows it too. He doesn't even crank his guitar like he used too.