How Do I Adjust A Subwoofer For Best Sound?

Head units are usually rated at peak power, (read that as downhill with a good tailwind) whereas an actual amp is rated as continuous, or RMS.
Also, there are different ways to get to 40 watts if you know ohms law. Power (in watts) is achieved from Amps X Volts. You can achieve a lot of voltage with a little IC chip, but higher voltage won't control a speaker as well. 2 Amps X 20 Volts = 40 watts
But 5 Amps X 8 Volts also = 40 Watts.
Amperage gets the work done. A fast and furious turbo Toyota may have 400 horsepower, but you ain't gonna put a 24' trailer on it. A speaker is a load.