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.
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