Small rooms are always the source of the a lot of issues, as the bass wavelengths are longer than the room itself. So is also our visually driven tendency to want to put a sub in a symmetrical location- half way between our mains for example.
So subs will excite some kind of room mode no matter what. The issue is what do we do to mitigate that. There are many solutions from bass traps to EQ. My experience is that using more subs turned down low is better than one big one. For example I would rather go into a small room with 4 cheap subs placed at different distances along the four walls of the room than one sub in corner.
I think Duke had the right idea with his swarm product. Also bass arrays work well, in small rooms or large, Bag End did a lot of work on that.
So we have more trouble with clients buying one sub that we do with clients buying two. Almost no problems with people buying 3 or 4. With ATMOS this is a big issue, as much of the basic ATMOS info says subs in front but that doesn't really work in practice.