The XBA 18 is a professional-class subwoofer loaded with one 18" woofer. It sports one onboard single-channel PWM power amp (class-D) with integrated control circuitry. These features let you adjust parameters such as levels, the upper frequency limit, and polarity directly at the XBA 18 without resorting to an outboard controller. If you wish to use the XBA 18 with a DDO- or controller-driven system, you can flip the Crossover switch to cut the integrated circuitry out of the signal path.