It seems to me that if one had a cabinet big enough for the size of many of the passive radiators used, why not just add a cross-over and add another woofer? Or just use a larger woofer to start with? Sure there would be some added expense, but I'd much rather have a proper sealed box than any ported design, passive radiator or not.
With DSP and digital EQ having made such strides recently, I wonder why someone has resurrected the eq'd sealed box designs like the earlier Thiel's up to the 3.5's with active eq bass boosting compensating for the natural roll off? With updated DSP and EQ one could adjust the bass so that not only in anechoic chambers but one's actual room, the most troubling frequency range, the bass, could be optimized.
True transmission lines are ginormous.