As a satisfied (very!) owner of four Bassmaxx ZR-18 drivers, I am of the opinion that the driver is a big contributing factor to the output capabilities of the system as a whole.
I've owned a large number of drivers from several manufacturers, and the ZR-18s are like no other on the market. Just sheer mechanical servo pistons with a very long stroke capability. In my vented boxes, two of them easily surpass 140dB. My boxes are essentially flat to 16Hz, +0/-1dB, as measured on a CEL 241/1 sound level meter. Maybe not quite as sensitive as some of the horns, but they go very low and with a monster amp behind them, they get the loudness below 30Hz. Due to the mass of the double-layer fiberglass cones, they don't seem as loud above 50Hz as some top commercial drivers with their distorted output, but get down below 30Hz and they really turn whatever building you're in, into one giant shake table!
Frankly, anything with a paper cone just doesn't do a clean enough job. Distortion is often mis-perceived as loudness. The Bassmaxx drivers have no distortion to speak of.
Where I find the most noticeable difference between the Bassmaxx drivers and other drivers is not in the room, but a block away from the building. It's the difference between a weak thump and the sound of one of those dual rotor Army helocopters hovering directly overhead. Clean bass really carries for great distances. Trying to insulated and soundproof against a ZR-18 is like trying to conceal an atom bomb detonation.