Here is my experience in designing and building a subwoofer. And I share my other project subwoofer filter but using IC 4558, Subwoofer Module 4558.
in this subwoofer filter, first at important parts is the active circuit filter 40 Hz, 12dB / Octave slope with a dual-ic opamp TL072 and several other passive components. The circuit details are as follows:
The subwoofer filter frequency response is as follows:
The PCB design and its layout: