Here the Univox SuperFuzz with the mods done by FSB member derringer. It replaces the stock tone switch with a pot, as well as a few other useful mods. Socket the 2 ceramic caps going from the base to collector on Q1 and Q6 and try some different values (I'm guessing in the 100pf to 500pf range). This one's a little on the wide side, so you'd be hard pressed to fit it in a 1590B, but it should fit quite nicely in a 1590BB.
