Getting into larger car and home systems, actual passive radiators are a bit more common, but still rare for the most part.
Maybe rare to 3R, but you should research Tang Band speaker modules. Many of us 2R O scalers use them. They are compact modules with an active driver and passive radiator built in. Some of them produce usable frequency response below 60Hz. Even their smallest model that I have used in HO and S will reach down to 100Hz.
