FWIW one scenario I’ve found stacked subs to offer potential benefit (as opposed to the aforementioned careful placement throughout a listening area) is that in which sub(s) must handle higher frequencies ( > 80 Hz ) if the mains will not.
If that scenario is not yours, then stacking subs in a line array approach should only only offer the benefit of each sub having less volume (excursion) demanded of it - that could be helpful for a single sub that distorts on bass-heavy tracks.