SUB-4K / SUB-5T Active Subwoofer High Quality LowPass Filter Freq. 40Hz~250Hz+Bass Boost+Phase - Motronix
Aiyima Low Pass Filter Bass Subwoofer Preamp Amplifier Board Single Power Dc 9-32v Preamplifier With Bass Volume Adjustment - Home Theater Amplifiers - AliExpress
Subwoofer special filter preamp tone board single-ended output | eBay