[collo]

You can also rip the bottom out of the entertainment unit and have a free-floating sub sitting on the floor. Leave a gap of about 1/2" between the side / top wall and the cabinet. Stuff the gap with a little bit of foam rubber, such as is used to seal around doors.

Mount the grille cloth on a frame that clips onto the entertainment unit. This will give the integrated appearance, but isolate the sub from the unit.