Dual 18' ' Professional Theater Banquet Sub Bass Audio Speaker High Power Passive Bass Reflex Subwoofer - China PA Speaker and PA System price | Made-in-China.com
Focal 100 IWSUB8 In-Wall High Power Passive Bass-Reflex Subwoofer
WTF is a Bass Reflex Speaker? - ecoustics.com
Line 501 Slim Sub Subwoofer Bass Reflex Speakers | active | 60W RMS | total power: 120W max | 20.32 cm woofer (8") | on 4 feet | frequency response: 50 - 100 Hz | impedance: 4 ohms | volume control | White