How Do I Adjust A Subwoofer For Best Sound?

Thanks for all the help, yall. It's much appreciated, even though I don't understand 90% or more of it. Just to make it clear what I have, here are the door speakers:
https://www.crutchfield.com/p_1131666S/Kenwood-KFC-1666S.html
Here is the head unit:
https://www.classiccarstereos.com/din-size-usa-4-cd-radio-chrome-face.html
Here is the subwoofer:
https://www.crutchfield.com/S-OP9jBNPeDZy/p_023THB210A/Blaupunkt-THb-210A.html
I also have this I "could" add to the front speakers, but looking at the specs for this and the head unit, I don't think there's anything to gain. The head unit is rated at 40w x 4 and the Kicker is rated at 30w x 2. So I think the Kicker won't do any good, unless somehow the head unit and the Kicker are rated differently.
https://www.crutchfield.com/S-twEXpxMpvgp/p_20641D1252/Kicker-DXA125-2.html
Lastly, here's a pictuer of the subwoofer and the small Kicker amps controls:
The subwoofer:
SUBWOOFER.jpg


And the Kicker amp. Any help is greatly appreciated.
KICKER AMP.jpg