Caution: Find out the output of your head unit (check specifications) and match to the gain setting on your amp. The gain setting is not a volume control. If it still is not hitting, you have other issues.
low level when using RCAs so that knob should be pushed out. The crossover turn it all the way right and use the one on the head unit and set it to 80 hz 12db. Turn bass boost off turning it all the way counterclockwise. Gain depends on your head unit voltage and where you set the subwoofer level control and source level adjustment.