Four 18's in a single enclosure? Srsly? First thought was the
JTR Captivator in various flavors, but those are dual-driver. Paradigm hexagonal subs? Those only go up to
6x 10-inch, for a total driver surface about 1/4 the area of four 18s. Multi-driver tapped horn, like a corner-loaded
Danley Cine-Monster? Maybe, but I'm not sure what those cost.
As for apples-to-apples, I suppose it depends on your restrictions. For output, vented subs are going to be more efficient than sealed, so the Captivator 4000ULF would probably be the most direct competitor. If you're looking for "musicality", I doubt you'll care as much after washing the blood from your ears off your shirt collar.
Is this just a bass porn thread, or are you seriously considering buying this nearly 300-pound monster?