agreed on not inverting the sub...and yes you are right you wanna see which one is louder generally speaking. if you shift the phase of the audio sine wave on the output of the amp by 180* you will change the time the sub hits to be the opposite of what it would be at 0* of phase shift. so instead of the sub excursing outwards at 0* it will be excursing outwards at 180*. so when ur sub is inverted like in a bandpass box or wutever the setup my be, it will sound more tuned to the beat.