Dedicated amp and sub would be my 1st upgrade, then sound deadening materials for the doors. Not a big fan of sound coming from the back as I like and truly enjoy the front stage. I am in the process of changing my sound system as well going all active. Picked up Alpine PDX V9 as the heart of the system to replace the factory infinity in the same location under the seat, Dynaudio for the tweets and mids. Have not decided yet on the subs. I am thinking of leaving the factory sub location and doing a fiberglass sub box on the right side of the trunk storage space, removing the foam and replacing with an enclosure for a JL slim 10 inch. Enjoy and keep us posted!