I haven't done this yet, but from my research the monsoon amp uses the speaker level inputs coming from the monsoon head unit (front L R, rear L R). Therefore if you replace the HU with an aftermarket one, you can simply connect the speaker level outputs of your new HU to the harness used on the previous unit, which will send the proper signals to the monsoon amp.
here's a link to the wire color codes:
The monsoon amp turns on when it detects a speaker signal, so you don't need to worry about running the remote turn-on wire to your aftermarket HU. As for power wiring, I'm not sure what you are referring to for the 75x terminal, and I honestly haven't looked into the power wiring yet.
Good luck and let us know how the install goes.