why use it? All it does is adjust the gain. You should find the max gain setting by turning the music up to loudest you will ever listen to it, and with gain on minium, turn it up until the sub starts to distort.
Any adjustments later on can be made by adjusting the bass freq/level/sub volume on the HU.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>