Figured I'd post this in this sub-section and not the How To, tech... no one will find it in there
I just had an inquiry about this from a member here and have in the past, so I figured I may as well post them here so I don't have to type it out every time, LOL.
Leave at zero. All this does is raise the response at some (usually unknown - most are around 40hz) frequency. Like an equalizer. This will turn your shit into a one note woofer... eh, you don't want that.
LPF Frequency Filter:
Most set theirs at 80hz. You'll have to play around with it, and see what you like best. Somewhere in the 60-100hz range though. The closer you go to 100hz, the more sensitive it's going to sound (because it's playing higher frequencies) and that will lower the power handling
of the sub woofer, so watch your gain.
This you'll want to match with your head units outputs. Check out what it's rated for. It'll be somewhere between 2volts-5volts (2v-5v). Just get it in the neighborhood of what your deck is rated for. Some amps don't have this feature, no biggy.
ISF (infrasonic filter) or SSF (subsonic filter):
This is the low frequency cutoff point (the opposite of the LPF). Go ahead and turn it on. Or, if you have the option, set it around 20- 25hz. Some amps don't have this feature, no biggy.
This is kinda of hard to explain, so I won't (lol), but every sub I've had in my Jeep I've had phase reversed (180 degrees). This is another thing you'll have to see what sounds best in your car. You'll notice one (either 0 or 180) will seem like it's 'lagging' or is 'step/beat behind'. Test each when sitting in the front seat and see what one sounds more 'up front' or 'on time.'
This is your, for all intensive purposes, volume control for your sub woofer. Set this to zero.
All other settings/knobs should be set BEFORE you fire it up. Start with the gain at ZERO.
To set your gain by ear; put a familiar song on with heavy bass (probably a rap track). Crank your head unit up to about 90-95% volume - music should be blasting. Slowly turn the gain knob up. You'll notice your sub woofer output getting louder. Keep turning the gain until you start to hear the sub distort or feel it's reached its max. Back down just a bit for good measure.
What ever volume you had your unit set at (example, 48/50), never turn your music up past that point (48 in this case). That's where you have set your sub woofers limit.
Let me know if you want me to explain anything in detail or differently.