I have changed the headunit to a Pioneer masterunit with a 7" retractable screen. I have a few questions regarding the amp of the system. My headunit has a power rating of 45 x 6, 4 channel for the speakers, one for center speaker, and one for subwoofer. Suppose I connect the 4 channel to my stock speakers normally and then add a subwoofer. How much power do I actually get from the headunit to the subwoofer ? I don't think 45w is enough, but it says it is able to run a subwoofer without an external amp. I am so confused !!