"This is a very interesting design, Alex. A 21” diameter driver is very impressive! (LL21 are you seeing this?)
What is the thinking behind the drivers facing each other but slightly offset? Won’t this result in some cancellation?"
Of course not look up mutual coupling. In that kind of arrangement at those frequencies you get an addition 3db so the net effect is a 6bd increase in output so you have just gained 6db which drops your power input requirements to 1/4 of the power you would need with a single driver so a win win all around. Used on PRO stuff all the time.
"I personally think there is some spatial or ambient soundstage or venue information well below 20Hz."
I agree there is some info down low there but there is a limit on how low. You can also get a lot of HVAC noise down there heavy trucks and subway NYC.
Rob