if iz low budget i'd suggest replacing d 2 6" and 2 8" with 2 audiopipe or powerbass mids speakerz i make a partner put 2 6" and 2 8" audiopipes with 4 pyle pro on a power acoustic gothic 1600.4 and dat was rel loud, u could bearly sit in the car when he crank it up.
2 12" category kinda hard to go low budget i'd try sticking with ur current baseline but get a better box built maby bout 2.5 cub ft. or consider replacing with a decent price sub many tuners have nice options like Exonic, Justbass, etc look around my personal choice is the CTD sub though
stating your budget mit help alot though