Hertz C 165 F Cento Series 6-1/2" flat-profile component woofers at Crutchfield
Carrozzeria TS-WH1000A 21 × Eight Cm Dual-sided, Double-sided Drive HVT Unit Subwoofer Built-in Amplifier | Built in AMP Sub Woofer Speakers | Croooober
Dayton Audio RSS210HO-4 REFERENCE Series, 8.3 inches (21 cm), High Performance HO Subwoofer, 4 Ω : Amazon.sg: Electronics