There are multiple "weak links" in the stock stereo system. The HU is far from great and the speakers are pretty much a joke. If you are looking to upgrade, you could add an amp and sub but your amps will be "line level" amps and distortion will be high. Your best bet is to replace the stock head unit, run an amp, sub, and new speakers. It can get fairly expensive. What are you looking for in the system you want to create?