If you have not replace your OEM HU, I suggest you re-install your previous OEM speakers to give a much better overall sound. You will have to replace the to at least an entry-level branded HU those below $400 type with a reasonable 6" passive component set to get the 'stage 1' sound improvements you're looking for. Stage 2 involves installing a 3-way 6 1/2" at the rear if you intend to end your ICE-experience at that level, if not than can go for 2-way 6" at the rear & wait for further budget for a stage 3 rear woofer & 4-channel amplifier installation.