Change the HU, put a 4-5 channel amp in. I changed the rear speaker for mis bass subs and deadened the doors. Made a huge difference. I will put in 6.75" components in the front with spacers (a must). The rear doors have a 5" depth (2012 WRX) so I went with 6.5" subs (free air) in the rear doors. Quite nice bass response, if you need big bass go with a stealth box and a good 10" sub (mono amp).