Subsonic filter is just a high pass crossover.
Subwoofer Phase Selector: Switches the phase of the low bass signal between normal and 180 degrees reverse and can make a difference in the sound according to the acoustic irregularities of the vehicle. This can improve the overall response of the system and bass impact.
Basically, with the phase, play around with it until it sounds good, and on each setting...close the trunk, don't leave it open...helps to have rear seats that fold down, but still can just run back and forth to set it and listen to it...
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Brando