The Crowther Prunes & Custard is a unique distortion box good for both guitar and bass. It's a two-stage circuit, built around a TL072 dual-opamp. The first stage acts as a preamp, and the second stage generates the distortion, with harmonic overtones. I've laid this out for on board pots (long leg PCB-mount pots will probably work best mounted from the component side) and it should fit nicely in a 1590B enclosure. The 2 switches are SPST.