I have a 12" rockford p300 powered sub running via high level input off of my front door speakers. Problem im having is head unit has only basic settings. No lpf or hpf. I would like to cut out the bass to my door speakers and have only low frequency coming in to my sub/amplifier. Normally i go with an aftermarket head unit so this is my first time using high level inputs. This amplifier does not seem to have too much adjustments either gain, xover, bass boost) Any suggestions or advice.