Greer Amps Tomahawk

 Here's a popular drive box from Greer Amps. It's essentially a Distortion +/DOD 250 but with a modified gain control, LED clipping and a treble cut pot. Should be an easy fit in a 1590B.

Hudson Electronics Sidecar

 Here's a really great sounding overdrive from Hudson Electronics. The Sidecar is loosely based on the Tube Screamer with a 2-band Baxendall EQ and some interesting soft clipping. It uses a PNP germanium as a diode along side an LED and a 1N4148 in an asymmetrical configuration. I've got a fabricated version available in the store as well if you prefer and the schematic is over there in the build doc.