The Passac pre-amps are great for Fishman-type piezo and crystal pickups. They add harmonic content that is missing. They call it "the sound of the soundhole". Juice Goose Company offered these a couple of years ago. Maybe not now. Quite a few are available "new in the box" from sellers who never bothered to figure out what they were. There is a belt-clip version and a rackmount (includes Aphex Aural Harmonic Exciter).