Subsonic / Rumble Filter for Phono preamps and Sub-Woofers
How to Set Your Subsonic Filter for Ported and Sealed Subwoofers – Stinger Off-Road – Jeep Audio And Electronics
Amazon.com: BOSS Audio BX55 2/3 Way Car Audio Mountable Electronic Crossover Bass w/Subsonic Filter and Wired Remote Subwoofer Level Control (2 Pack) : Electronics