Oh, that...
That was a test, which I never want to repeat again. I had such a mess to clean up after that from dust that fell from the ceilings and from things that fell off shelves and broke--all over the house. Thanks to Bassmaxx Technology and CGN Audio Labs for those prototype subwoofers. Mine toured the Nevada and California deserts at "raves" before they changed ownership, where they tested their mettle with 9,000 watt QSC Powerlight amplifiers bridged mono. That YouTube video was quite tame, actually, with probably about 60W going into each driver at the time, guestimated by the -30dB LEDs flickering on the amplifiers driving the ZR18s, producing not even an inch of excursion. The Hemholtz resonators do most of the work below 20Hz.