FIG. 1 is a perspective view of a subwoofer showing my new and ornamental design, FIG. 2 is a front view thereof, FIG. 3 is a rear view thereof, FIG. 4 Is a first side view thereof, FIG. 5 is an opposite, second side view thereof, FIG. 6 is a top view thereof; and, FIG. 7 is a bottom view thereof.