The only passive radiators I have seen and heard in a few models were incorporated in to small powerful subwoofers to attempt to reinforce the bass in ultra small cabinets. Despite the claims of deep, powerful bass in a small package, I was never impressed with any of these overpriced designs and I think the marketplace has shown it.
When it comes to subwoofers, I have always believed that you can't really defy the laws of physics, the bigger the cabinet, the better, deeper AND louder the bass with less distortion. Since it will give somewhat more flexibility in placement and provided it is competitively priced, the bipolar design is one that Axiom should strongly consider for the future, forget the passive radiators.
Casey. My understanding of subwoofer design is that you use the passive radiator to eliminate the internal volume of a port (making the sub a little smaller) and to completely eliminate the issue of port chuffing. The cheating of physics in small commercial designs is probably a design compromise where the designer wants a smaller cabinet more than deeper bass and so reduces cabinet volume beyond just that which the port makes up.
