You've got most of the facts on this guitar. Bob Weir and Phil Lesh mostly played Modulus during the 80's into the 90's. Bob Weir was a major investor in Modulus during the 90's. The unusual pickup placement was inspired by Bob's main guitars which where through-body. The active tone control is probably an EMG SPC which is a presence/mid-boost control. EMG had a number of different models that all has similar configuration (one is a midrange cut, another is a straight gain, etc). It will say which one it is on the PC board if you look closely for markings.
If you look closely at the EMG logo on the pickups, you may see that they are different colors. Back then EMG coded the models using different colored logos. A silver logo is an EMG-S, a gold is EMG-SA, and there are white, blue, and red models at well which all have different coils and built-in EQ. Some of these guitars with the three close pickups are built with EMGs that are custom wound for Modulus. They sometimes have handwritten stickers on the back of the pickup. If there's a printed label with a production code back there, it's a regular pickup.
I remember seeing some of Bob's guitars at Modulus in for fretwork back in those days. Some of the later ones had some very unusual stuff - there were blocks of rare metals that were inlayed into the back of the headstock, Roland controllers, etc.
David Fung
