I’ve purchased one of your J-Retro 01 preamps for my fretless ’98 Mexi Jazz, and installed with no problems.  I’m using Lindy Fralin split-coil pickups, with a 5% overwound bridge pickup. Oh my…I think this just became my favorite bass! Those Fralin pickups have all sorts of growly tones buried in there, and the preamp brings them out VERY nicely.