[Schulisco]

+1 on the replaced speakers if they're too old. Old car speakers oftem may have paper cones, they get soft due the humidity over time. Modern speakers often have either plastic cones or plastic coated paper cones. But the most bang for the buck brings one of those small active subwoofers as already mentioned. With them it doesn't matter what kind of head unit you have. Also make sure that the cables are thick enough and the contacts are good and clean.

Hint: I recommend to use a subwoofer with a remote control. You'll like to have one to manage the power...