Subwoofer for my amp?

If you have connected to the same amp the front , rear and the sub, you should get a second amp because it will be to much of a load to the amp, That will cause the amp to overheat, to work near or ped its limits causing to get damage. also get a capacitor since having 2 amps will dim your car's lights whenever loud b is played. If only the sub is connected, then you're wasting 3 good channels. The best recommendation is to get the same sub to avoid differences in the sound.