And that's a problem--conventional 18" drivers are useless at producing anything but harmonic distortion below 30Hz. Same problem with every commercial driver that I have applied to the task over the years. Only the new long-throw drivers with specially built suspensions produce any meaningful output in my tests. That's why the ZR18s broke new ground in my quest for subsonic bass. I just wanted to get clean organ pedal tones at very high volume, but ended up getting a demolition machine in the process.
It's more fun to achieve "brown notes" with technology, rather than by using non-audio methods, as you described. I am motivated by those types of challenges. Anyone can build a valve and a concrete chamber, but build a subwoofer that can also play music and that's heaven!
I had a BBQ party last Sunday and some very wealthy audiophiles were in attendance. None of them were anxious to see this system go over "club levels" and as such, I never got beyond illuminating "signal present" LEDs on my QSC PL amplifiers. But later on, one of the audiophile guys, a gentleman responsible for 'poisioning' me into becoming this Bass Pig some 30 years ago with an exposure to his big system, informed me that the demo today had a very effective laxative effect.
Now I have a new sonic demonstration weapon in my arsenal: A hand-restored Telarc "1812 Overture" digital recording, in which I restored all the badly-clipped cannon shots to their full amplitude. Then I have the audacity to run that through the dbx 4bx Expander with Impact Restoration, for just a tad more impact than the original.
I won't play that for people without signing and indemnity waiver.
As for RTAs, relatively limited value, in my experience. This room has a big problem with bass absorbtion and cancellation, which is why the system grew so big over the years. The place is a black hole for bass. An ordinary set of stereo speakers in here sound really bad, with no bottom end. The room doesn't get excited and hence it seems like it's playing in a vacuum.