B&O Tech: Subwoofer Tweaking for Beginners – earfluff and eyecandy
500 hz, 12db/oct High Powered Low Pass Filter (Crossover) for Subwoofer Enclosures | Reverb France
72-130Hz AC 9-12V Subwoofer Low Pass Filter Subwoofer Electronic Crossover NE5532 Subwoofer Preamp Electronic Frequency Divider - AliExpress
Amazon.com: PRV AUDIO Low Pass Crossover 1DF180B Low-Pass Filter for Speaker and Subwoofer Crossover Point at 180 Hz with Unique Selectable Features : Electronics
SUB-1STR Single DC 12-24V Active Low Pass Filter Crossover Subwoofer DIY KIT - Motronix
Amazon.com: PRV AUDIO Low Pass Crossover 1DF180B Low-Pass Filter for Speaker and Subwoofer Crossover Point at 180 Hz with Unique Selectable Features : Electronics