You should, and if you do, you'll get a broadband base trap. Hopefully, you sealed the riser completely. Place at least three registers along the perimeter, very close to the walls. One along the back wall, and one on each side wall, biased toward the back. My registers are pretty big (4x36). The concept works well, but the riser should be sealed so that the perimeter ports are handling the airflow. I also put three layers of plywood down with green glu in between the layers.