"And thats where the coaxial and cardioid elements of the speaker come to play, less room dependant."
I agree it’s something people repeat a lot, but it’s only partially true. In a typical 15‑square‑meter room, the Schroeder frequency is around 200 Hz. And below 200 Hz, almost no loudspeaker is actually cardioid. On top of that, it’s important to understand that treating the highs and mids is relatively easy : with 10 cm of dense rockwool, you still get an absorption coefficient of 0.5 or more between 200 and 20,000 Hz. At 40 Hz, using porous absorption, you would need about 45 cm of attic‑grade insulation to reach the same coefficient, which is simply impossible to implement in most rooms. You can of course do better with membrane bass traps, but the calculations and the practical implementation become far less trivial. So, yes, controlled directivity do help, but not necesseraly in the frequency range were it's the most difficult to handle. Multi subs can also help to make the room modes more even at the listening position, but won't adress the time domain issues either.
"Interesting point; it's really not. There is some of the fundamental 27.5Hz in there, but harmonics are dominant (>100% of fundamental)." We're on the same page.