The JBL sub outputs are already at preamp level. Two of the 4 pins are 12 volt power to the JBL sub's amp. You could conceivably solder on your own RCA's to an aftermarket amp, using the 12 volt as a remote turn-on signal for the amp.
I already knew they were at pre out levels. I have though of creating my own and I am searching for that elusive harness. Do you happen to know what the wires are? I have also had the idea of just soldering directly from the amp without the harness but don't know what each pin is for.