So I have an Audi A3 8p 2004, which has the standard concert system (not bose) HU is also a concert 2. Today I have installed an AUX whilst still having the original player. What my question is, I still want to have the original hu, and want to upgrade the sub and maybe the door speakers. But i am not sure what to buy, what sub is going to fit, what is compatible and how to set it up. Do someone here have any noob guide?