What I have found is when the main speaker radiates bass in a figure 8 pattern, like here, then you really really want to have the sub match that pattern. Box subs stick out like a sore thumb because they radiate like a pulsing sphere. Makes perfect integration almost impossible.
Has not been my experience and I cross over well above 100 hz, but I may not be as fussy as you with all of your show experience.

That's not a dig, but an honest assessment.
The sub used is probably more important than the pattern, and also the implementation would be critical. I would also say it has to be acoustic suspension, or sealed, to blend in.
Rocket Ronny