I think you have raised the fundamental issue. The crossover between subs and the rest of the system, and especially the mains, is to all intense and purposes an "off the shelf" crossover. The likelihood of a bad result is much higher than a good one. If you have three way main speakers, when you add a sub you have a four way system, and if two way mains you have a three way system. That means speakers and sub need to be designed as a single system from the start. That has always been my approach, and it is an absolutely crucial issue in my opinion.
Sigberg audio seem to agree. They are designing their subs as a total system. That is the correct approach.
Look at their sub integration. The crossover execution is superb.
I find the Sigberg design approach refreshing. In many ways, it is because they a going down many of the same roads I have travelled for years, sans the cardioid venting. However to return to the issue. That is how the cross to a sub needs to look!