Here's a pretty simple but effective bass preamp from EWS. It's a 3-band EQ being pushed by a MOSFET gain stage. Looks like EWS made a version for Xotic with the same name. Not a lot different between the two other than some value changes laid out in this schematic on FSB. I've laid it out for a 125B with top mounted jacks. It runs off an 18v supply so use your favorite charge pump if you want to include that in the pedal.