For JT’s question, not the answer one would hope for:

The beginning of the answer is that the ideal model for a +3dB across-the-board rise is an anechoic environment where two subwoofers would be co-located, but none of us lives in such a situation. Separate the subs by more than about 10% of the wavelength of a frequency in question and place these subs in real room and you’ve got the complex situation you mention where sophisticated computer modeling can’t exactly predict real world results and experimentation and measurement become the means of pursuing the best results, even for true experts. Suffice to say that the lower frequencies, say 30Hz and down, from dual subwoofers in many real world environments, will tend to reinforce each other more so than the frequencies going upward toward 80Hz, which tend to experience more of a smoothing/leveling effect when coming from multiple sources.

The middle of the answer lies in that zone you mention: too complex for us now, at least in these posts. I got my start in the mid 70’s with a book called Sound System Engineering. The current edition is 665 pages and has almost no ‘easy answers’ that would satisfy what is sought in this post. While some of what’s pictured in this book may seem outdated, the principles hold true regardless of the era from which a particular piece of equipment originated. If you have an aptitude for this kind of material, look in books or online.

The near-end of the answer, if you don’t want to spend a lot of time learning the middle, doesn’t have to be difficult if you’re willing to have a some trust in those that have studied, carefully modeled, tested, measured and evaluated aspects of at least part of the question at hand. Their answer: used properly, two of a given subwoofer is better than one (for several reasons beyond a simple +3dB gain).

This doesn’t answer questions with budget factored in, such as: which is better, two $300 subs or one $600 sub? Two $600 subs or one $1200 sub? The only way you’re going to know which will be better for you in your environment is to audition both solutions, taking the time to carefully set levels and try for the optimum placement of one sub or a pair. If that’s too difficult for one reason or another, you’re back to weighing research and opinions and taking your best guess.

Opinion time: I tried it both ways. I now have two subs, not for bragging rights but because the dual sub setup sounded and measured better once setup to my satisfaction. At the end of the day, those that have differing opinions don’t have to live with my choices, but I do.

Poll time: among those that have tried one and tried a pair, how many have stayed with two subs and who has gone back to just one? If you went back to one, was the decision primarily financial, or because you found little or no better results with two? Or for some other reason?

OK, I’m going to try and stop writing about this now. I’ve said my peace. I hope some find it helpful.