Seymour Duncan Dirty Deed
Here's a pretty interesting distortion from Seymour Duncan. 2N3906 transistors for hard clipping and a two band gyrator EQ section. Originals I assume are all SMD and use MC33178 ICs, which I believe are equivalent to the LM833, but other dual opamps should work fine. Schematic here. Should be able to squeeze it in a 1590B with small profile jacks.
Run Off Groove Mockman 2.0
Continuing to knock out the last few ROG designs I haven't laid out. This one's a pretty easy little circuit, emulating the distortion in Rockman amps designed an used by Boston guitarist Tom Scholz. The toggle allows you to go between 80s (classic) and 90s (modern) tones, and it should all fit in a 1590a. Schematic, sound clips and more info on the ROG site.
Wampler Cranked AC
Another amp emulation overdrive from Wampler that's supposed to sound like (you guessed it) a cranked Vox. This one has been discontinued, but was very popular. It's very similar to the Plexi-Drive, but doubles the transistors and there are a few component changes. I've added trimmers to help bias the J201s.