You could see if BSW offer an amp upgrade, with OE connectors.
I replaced the front door 4", added tweets, replaced underseat woofers, with OE plug n play connectors and wiring from musicarnw.com. Output from headunit is connected to JL Cleansweep processor/equaliser, from there to amplifier and back to speakers.
I left the rear doors as is, they have little or no impact on sound quality.