Best settings for kicker amp?

LPF does exactly what it says -- it passes low frequencies (implying cutting high frequencies).

They are beneficial because a subwoofer cannot accurately play high frequencies and will distort trying to.

There's no "correct" frequency to set it at, but 80hz is a typical baseline. You may prefer it a bit higher (90-120?) or a little lower. It won't hurt to experiment.

Gain needs to be set in relation to the signal strength of the head unit. Search for "gain setting tutorial". There a many out there.

Any bass boost is usually too much.

basically the more gain means its more sensitive it is. the bass boost will just increase the bass by making it louder, so set this to a personal level of amount of boost and gain. usually you turn the headunit to 3/4 high and turn up gain till sounds like clipping, then turn gain down a tad. frequency set i think you are mean low pass/ high pass filters. this means basically when its on, the low notes go to sub, high / mid go to speakers. this is what i find greatly increases the sound quality overall

Set the x-over to LPF @ 80hz.

The gain isn't suppose to be set randomly. It's suppose to be set using a DMM, an o-scope, or just by ear.

Turn the bass boost off.

Here, watch this video series: https://www.youtube.com/watch?v=WhrDqke8BKo. Or take it to a pro shop and have them set it up for you.