Here's a simple bass preamp circuit derived from the Sunn 200S using JFETs. It's pretty similar to the Bassman Preamp, but with the EQ section at the end of the circuit instead of the middle. Original S200S amps didn't have the Mid control, but it's kind of necessary IMHO. Bias the JFETs to 4.5v and adjust to taste. Not sure if the BAT85 diodes are necessary as they're not in the Bassman Pre, but I left them in just in case. Also not 100% sure if the below demo follows this schematic exactly, but it should be in the ballpark. Should fit nicely in a 1590B.