How "aftermarket" do you want to get? If you want increase in bass, just use a "high-low" adapter with your rear speakers to amp/sub setup, if you're looking to change out everything including deck, and speakers, that's going to be a tough one as a deck will only power 4 speakers, on top of that, your monsoon system might have different "ohm" speakers that would potentially blow your deck- so you'll have to get new speakers. Plus, you have 7? 9? speakers, so you'll definitely need another amp as well to power those other speakers.
