A Passive Radiator is ...
Basically a driver without a voice coil or magnet structure. they were used in sealed cabs to enhance/tighten up the bass. Although the boxes were sealed/airtight the P/R did. in effect, act as a 'port'. finicky to tune for the home builder, because they required adding weights to the cone to get the right result. Polk and a host of other mfrs used 'em in the 70's and 80's. Most of 'em looked like ordinary speakers, but some of them were flat panel with a surround that looked a lot like that KEF. Hence the confusion on my part.
Mike