Mackie brought 'back' the idea of a passive radiator coupled to a tuned cabinet to augment lows with their first series of powered studio monitors (and Mesa also used them to great effect in the old Walkabout combos). The Mackies sounded REALLY good, which is why I don't like them for studio monitors. They're almost hi-fi stereophile in their exaggeration of critical areas where in the studio, you need accuracy, not beauty. I'd track with them, but not mix or master using
A passive radiator is basically a driver with no magnet or voice coil tuned to react at port frequencies and below. If you've ever been playing and something in the room would vibrate sympathetically, it's the same principle. Since there's no coil mass (because there is no voice coil at all in purpose-built radiators), the suspension can be tuned as tight or as loose as needed to get really huge lows out of smaller diameter speakers. The downside is that unless the port is carefully physically designed, those extended lows can be out of phase to the rest of the other bass frequencies and it becomes progressively worse below the port corner frequency. Since bass wavelengths are so long (around 27 FEET for low E depending on temperature and altitude 767mph/ 41.2 Hz) it can be hard to hear, but when it's a problem, it's a MAJOR problem, which is why passive radiators are not more common. They make perfect sense for an instrument amp being powered from a single source, but when you start adding multiple cabs from multiple amps, things can get weird in a hurry.
Part of the irony of speaker science is that we are still in the second generation of driver development for most bass speakers, and the exact same principles that modern drivers use were first employed in the first permanent magnet drivers of the 1930's. There have been forays into pushing the state of the art: servo-driven subwoofers, ribbon tweeters, and a few other esoteric designs, but by and large, what little gains in speaker efficiency have been made in the past 80 years have been in the refinement of materials and cabinet reflex designs. Even the BEST subwoofers are barely 20% efficient; everything else put into them is lost as heat. If we could make drivers with an efficiency of 60% or better, high powered bass amps could literally be used as sonic weapons. That's also why it's always going to be easier to get more volume with more drivers instead of more power.