The original system stopped working, the owner was interested in something else. An in house FET powered system was made and mounted in a cover plate that suited the highly figured wood the guitar was made from. imho.
The knobs were made from walnut/mahogany offcuts using a jig with a plug cutter. They were then turned on the drill press and a steel pin indicator was inlaid into them.
While the FET is old school I like it's tube like qualities it plays well with others and is great at warming up piezo pickups.