Best system to replace monsoon with???
If you just want some more cabin shaking bass, you can keep the setup as you have it and just add an amp to power a subwoofer box or tube. The headunit you have should be able to send a low level wire or RCA signal out to the aftermarket external amp for the subwoofer.
You almost need a set of single channel bridgeable amps , OR a monster amp that can handle low resistance speakers...
So easiest way to get loud is a new amp and new speakers... "Some assembly ma be required" on fitting aftermarket stuff in the factory speaker grills.
Been 2 decades since I messed with one so just a broad stroke opinion..











