Just to add some clarification, PR and port based subs are both bass reflex designs whereas a "sealed" sub is a acoustic suspension design. A PR is weighted for tuning (the heavier the PR, the lower the tune) and has no motor structure. One issue is that the PR(s) need to have a minimum of 2x the Vd (volume displacement) of the active driver, with 4x the Vd being even better.