The ST21-4, a 21" subterranean subwoofer was developed for high SPL bass output outdoor applications. It features a powerful 21" transducer for maximum excursion and SPL. The ST21-4 can be directly driven by an M1000 subwoofer amp which produces 70V of output, or 1000W+ at the 4ohm rating. However, at 1000W, wire loss needs to be considered and calculated as with any other 4 ohm wire run. In all applications, the ST21-4should be bi-amped.