BBE Boosta Grande

The Boosta Grande is a simple opamp-based boost that's pretty similar to the MXR Micro Amp. Only differences being where the pot is in the circuit and component value changes. Offers up to +20dB of boost. Should fit in a 1590A if you don't board mount the pot.

BBE AM64 American Metal

Here's the BBE AM64. While "Metal" is in its name, it seems more suited for heavier rock sounds. 70s metal at best. Nonetheless, it's a very good sounding distortion box. The circuit is essentially a Guv'nor with soft clipping instead of hard, along with a DS-1 tone stack.