Here's another circuit from Xotic. Simple JFET boost with an output buffer borrowed from the Tube Screamer. Originals have 2SP1815 transistors with BCE pinout, but I've laid this out for the more common 2N5088 with CBE pinout. The originals also have the Bright and Fat switches internally as DIP switches. Having these externally should be much easier on the user, and it shouldn't be too hard to fit in a 1590A. Well, no harder than any other 1590A layout.