Two major aspects are never addressed by manufacturers: the clarity and intelligibility of high power bass. The quality of the result is ensured by separate transducers for the bass drum and the bass guitar, possibly for the low frequencies of the keyboards. No matter how good the research, this fundamental aspect is overlooked, while it makes all the difference between a clean sound and a crap sound.
I used to have a lot of fun playing around with stage layouts with MTL1X subs about a decade ago. Different delays for each cabinet, occasionally swapping phases and directions of cabinet placement. Whatever we could do to keep the sound moving forward away from the performance without feedback pockets on stage. With 8 cabs per side, every day was a science experiment.
It's crazy the technology that has come along to do the stupid human tricks for us lol.
That one design was just a 6th order parallel-tuned bandpass. Interesting. I've been mulling over an interesting design in my head that, thanks to this video, I now think would have some degree of cardioid effect. It's basically two TQWT speakers slammed together with isobaric drivers. One tube would be tuned to the bottom of the desired range with the other tube tuned to the top. The differing lengths of the tubes would create some natural delay between the signals and the ports could be arranged to help accent whatever change in the acoustic center was caused by the different efficiencies of the two tubes.