FIG. 1 is a perspective view of a subwoofer according to our novel design; FIG. 2 is a top view thereof; FIG. 3 is a bottom view thereof; FIG. 4 is a front view thereof; FIG. 5 is a side view thereof, taken from the left of FIG. 4; FIG. 6 is a side view thereof taken from the right of FIG. 4; and, FIG. 7 is a rear view thereof. The broken lines are for illustrative purposes only and form no part of the claimed design.