It's the system's woofer, but not a subwoofer - the crossover point is 220 Hz. It's a great sounding system though.
I used to own this system. The mixer is fine, but better suited to a live musician who's going to be less concerned with cable management and appearance. For me, having it built into the speaker was cumbersome - the controls are packed together tightly and it's not the easiest to navigate when it's just 2 feet off the ground. All of the functions are built into an app as well, but that's not any easier to navigate. It is, however, nice to have. On a scale of 1-10, it's a solid 5.