DS18 SQBASS8 8" Under Seat Amplified Powered Car Subwoofer 800 Watts
MT21A Built-in DSP single 21" Self-powered Subwoofer with Compact Cabinet Box - Buy 21" subwoofer, single 21 inch subwoofer, self powered subwoofer Product on Sanway Professional Audio Equipment Co., Ltd.
12" single Compact High Performance Glue it & Screw It Sub Enclosure - GP Car Audio
JBL JRX218S 18" Compact Passive Subwoofer
Infinity BassLink SM 2 8" compact powered under-seat subwoofer enclosure at Crutchfield