I tried a bunch of things- including crazy shielding, a few different switching ideas and finally sending it to Graph Tech to have them look at it.
In the end I wasn't happy so I replaced the piezo/mag quickswitch with a 4PDT mini toggle. Pos 1 gives me passive mags and disconnects the mags from the input of the preamp so there's no loading- exactly what you'd have w/o the piezo. (Also handy for when your battery dies on stage.) The other two positions allow both active mag and piezos ("both") or ground the active mags for piezo only.
I can do one up when I'm back in town next week for sure!
Sorry for the delay (and reviving an old thread ...). Finally found my wiring notes underneath a big pile of drywall in my basement-in-progress. I'll shoot those out to ya.