Phase shift is used to strap two amps togather. If only one amp then set it to zero. Variable low pass filter is the crossover frequency for the subwoofer. It cuts out frequencys over the set frequency that you dont want the subwoofer to produce. The Variable sub-sonic filter is for use with a ported box. It will cut out frequencys below the set frequency. If you dont have a ported box then turn the sub-sonic filter to the lowest frequency. If you do have a ported box then set it about 10hz lower that the boxes tuned frequency.