[08srtMA]

Hey guys. I dont believe I have the premium sound (atleast the speaker plate is blank I have an 08) but I do have NAV. I want to upgrade my speakers with something good not to expensive but better than stock. What brands to look for? I want bass but do not want to put a sub in the trunk. How are polk audio speakers/ infiniti? The reason I ask is because I have a gift card to BestBuy that I havent used and would like to maybe get something with it. I have a 12" JL sub and box but like I said dont want to put those in. Any help. Thanks