SUB-1STR Single DC 12-24V Active Low Pass Filter Crossover Subwoofer DIY KIT - Motronix
Amazon.com: KNACRO DC 10V-24V Low-Pass Filter NE5532 Bass Tone Subwoofer Pre-Amplifier Preamp Board : Electronics
SUB-2 Active Subwoofer LowPass Filter High Quality Freq.40Hz~400Hz+Bass Boost+Phase Control - Motronix
Single Power 10-24v Overweight Subwoofer Front Stage Plate Low Pass Filter Board Front Stage Tone Board Front Finished Board - Integrated Circuits - AliExpress