Best Buy: JBL 10" Dual-Voice-Coil 6-Ohm Subwoofer w10GTiMKII
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Onkyo W 20178 D 8 Inch Sub woofer 50 Watt 6 Ohm Svc | eBay
Speaker SB Acoustics SB42FHCL75-6, impedance 6 ohm, 15 inch
Amazon.com: ASHATA 600 Watt Car Subwoofer, 6 Inch Car Speakers, 4 Ohms Car Subwoofer Loudspeaker Midrange Coaxial Speakers Black for Car Audio Systems : Electronics