The Mackie SWA1501 is an Active Subwoofer with integrated electronics designed to control system performance.
Built with the idea that subwoofer performance should focus on the "attack" of the sound, Mackie has incorporated design features such as the double horn-loaded chambers, which give a significant increase in volume output while lowering distortion. They've also matched the 15-inch woofer with a powerful 500-watt amplifier capable of maintaining high-current levels at lesser sound levels. This allows for overall better control of the woofer.
The SWA1501's amplifier is mounted on a huge heat sink for cooler, more efficient operation and is managed by an electronics network capable of handling the protection, equalization and crossover filter of the system.
If you're thinking of using the SWA1501 in a portable PA system, you'll appreciate another nice touch, the high-pass output of SWA1501. This output allows the subwoofer to be used with other PA systems in need of more pronounced lower frequencies, and with its side-mounted steel handles, system portability is easier than with other systems offering less performance.