As the title suggests I am in the process of building the preamp section of the Maestro EP-2. Two of them in series in fact with a switch to chose between running just one of both pre’s in series.
Cool! For S1A and S1B, I suggest wiring the 470k directly to ground and then have the switch connect a 120k in parallel. The end result will be about the same, but it will eliminate the pop when switching.
Ditto as Thomas says - plus - depending on what pup you use - you might consider a 2,2M in place of that 470k - them piezos like high input impedances of next stage. Naturally then you would have to change the 100k pot to 500k or evev 1M to have it have any usable range.
For electric guitars original is perfect - except what Thomas said.