Kicker HS12

12" hideaway active subwoofer

Compact Powered 12-Inch Subwoofer System – Low-profile, all-aluminium 12" powered enclosure designed to fit under seats and in tight spaces while delivering strong low-frequency impact.

Not exactly an underseat sub, but it is still compact enough to fit there in some vehicles. 

Also ideal behind seats in utes, even under the floor in small hatchbacks or SUVs.  

• FIT+ Integration for Modern Vehicles – KICKER FIT+ (Fail-safe Integration Technology PLUS) accepts input signals from 0.25V to 40V (up to 400W at 4Ω) and is compatible with many newer start/stop vehicles without requiring an external line-output converter.
• 230 Watts RMS with Full Control – Built-in 230-watt RMS amplifier with a 12 dB/octave low-pass crossover adjustable from 50–120 Hz, a 24 dB/octave high-pass filter at 25 Hz, 0°/180° phase switch, and a variable KickEQ bass boost up to +6 dB at 40 Hz.
• Included Remote Bass Level Control – Wired remote knob lets you adjust subwoofer level from the driver’s seat without changing the main radio volume, so you can dial in the right amount of bass for any song or volume setting.
• Broad Frequency Response in a Slim Enclosure – Plays from 25–120 Hz in a compact chassis measuring approximately 17-1/8" (D) x 12-1/8" (W) x 3-1/2" (H) and weighing about 7.92 kg, making it suitable for a wide range of vehicles and seating locations.