Never owned a car with a custom sound system. Recently bought a car which has a prewired (working) amplifier Alpine mrp-mp350 700w in a trunk. It is wired to the stock audio head unit with a seperate switch to turn on/off. And a cable hanging for a subwoofer +/-. So Instead of taking it out, decided maybe buy a subwoofer but have no knowlege how powerful sub I can connect to it. Can somebody recommend something for the first time buyer/user?