If this number is lower, the value of C1 needs to be slightly increased. It is advisable to open a full subwoofer amplifier volume and adjust the sound level P1. This ensures that the input of the amplifier subwoofer can not be overloaded or damaged. Make sure that the soil of the signal lines associated with the base loudspeaker amplifier subwoofer. If the phase reversal is required, this is best done by reversing the cable to the subwoofer. If in spite of the additional protection above the desired at the input of the amplifier subwoofer, is most influenced by the 'overload protection' elsewhere on this site.
See also active filter and booster subwoofer :
Subwoofer Booster Circuit 4558
Subwoofer Module 4558
Subwoofer Active Crossover