PreSonus Temblor T8 Studio Subwoofer. 
Active subwwofer

A solid foundation for mixing.

A subwoofer provides the extra-deep low frequencies you need when mixing on relatively small monitors, especially for music that requires serious low-end punch. Powerful and musical, Temblor® T8 active, downward-firing subwoofers are designed to deliver fast and accurate transient response, extending low frequencies without a hint of “tubbiness” or overshoot. This ensures that Temblor-series subwoofers complement full-range monitors without overshadowing them, naturally extending the reach of your full-range system’s low end so that you can produce your best mix from top to bottom.