The ZAPCO ST-500XM III is a single-channel Class D mono bass amplifier designed to provide dedicated power to a subwoofer. It delivers a flexible range of output depending on your setup, providing 400 W at 1 Ohm, 300 W at 2 Ohm, or 200 W at 4 Ohm, all while maintaining an efficiency rate above 85% to keep energy consumption low. Its frequency response spans from 10 Hz to 200 Hz, ensuring a deep and consistent low-end performance with a signal-to-noise ratio exceeding 85 dB and total harmonic distortion kept under 0.2%. To help you fine-tune the sound to your vehicle's acoustics, the amplifier features a comprehensive set of variable controls, including input gain, an electronic crossover, a subsonic filter, and a bass boost with an adjustable center frequency. You can also manage the phase to align the bass with your other speakers. The unit is housed in a sturdy aluminum chassis measuring 160 mm wide, 58 mm high, and 225 mm in total length, making it compact enough for various installation spots. For added convenience, it comes with a dash-mount remote gain control so you can adjust the bass level from the driver's seat. Interestingly, these amplifiers can be strapped together in pairs to power a single voice coil, offering more versatility for advanced audio builds.