Unfortunately it's really not that simple. You don't have to look far to find audiophiles who claim they can't cross their sub over above 60hz or it becomes "localizable"(total nonsense). Nor to find those who prefer no sub at all, or are confused and think REL subs are more "musical" when all they provide is less bass, so that screwed up bass doesn't sound as bad since there's.. well.. less of it. A whole business built on misconceptions about subwoofers. If your single sub is affected by room modes sufficiently different from the speakers it's crossed over with, it can stick out in a "localizable" way even though those frequencies by themselves aren't localizable. Vibrations and harmonics can cause the same issues. A poor crossover can create a null due to phase problems. A poorly optimized sub can sound worse simply because bloated low bass is worse than none.You won't achieve the score presented without a perfect sub, but any satisfactory sub will get them a hell of lot closer than they were before. I agree with multiple subs are better, but like everything else there are diminishing returns and it won't be twice as good. IMHO it will always require manual tweaking. If you have access to both auto calibration and manual that's the best, but I'll take a Umik 1, REW, and trial and error over Dirac live any day.
There is another disadvantage of the single subwoofer beyond integration. It reduces you to 1 bass source. Stereo speakers provide 2 bass sources. Flexible positioning can compensate for this, but that requires you to actually consider and measure multiple locations. How many setups have you seen where the sub is positioned right up next to one of the speakers at the front of the room? I've seen many. This is another case where a sub can be worse than no sub.
2 bass sources help a lot with all of these issues, especially for those not using EQ or inexperienced with it. For example, here's an old and very illustrative post showing the difference between 1 sub and 2 subs *without* DSP. Later on in the post DSP is added, but the response is surprisingly decent when 2 subs are used even without EQ. And you definitely can't achieve what Dirac Live Bass Control or Multi-Sub Optimizer accomplish manually. It's physically impossible. They go through thousands of different settings to find the perfect combination.
I don't want to turn this thread into the sub thread, as we already have one of those. But it is indeed possible to make things sound worse by adding a sub, and I think it's more common than many would admit. Unfortunately, they usually blame the sub, which is not the real culprit.