I think your mistaking LPF with a bass knob...
A bass knob will turn just your subwoofer bass down like if you pass a cop or you dont wont the bass so loud and you can mout that just about anywhere on your dash....
The gain (level) is the most important.
The purpose of the gain is to match the signal volts RMS coming from the source (CD player, etc.) to the input of the amp for correct power matching and to prevent clipping (damage by over powering).
Set the LPF to just below the highest range of the frequency response of the subs. Meaning if the frequency response of the sub is 20-200Hz, set the LPF to about 180Hz
OR
set the LPF to where the other speakers start off. example, if the speakers start at 150Hz, set the LPF to about 160Hz. This overlap (or crossover - where the term comes from) is the ideal setting.
Set the subsonic (infrasonic) filter to about the same as the lowest setting in the range of the frequency repsonse of the subs. Meaning about 20Hz if that's the lowest in the range (20 -200Hz as above, if applicable).
Don't set the Bass Boost above +2dB (if applicable).
Set the phase to 0 (if applicable).
