Long thread but I will add my 2 cents. My room is quite small. Although it is irregular, the main listening space is 13X19x6.5ft. Yes, 6.5”! Basement man cave. No windows, heavy shag carpet. Some absorbers hanging on the walls but no room for bass traps or ceiling panels. System is on the long wall, with speakers out 2 ft from the front wall, tweeters 6 ft apart, and 8 ft to the listening position. Mains are Totem Element Metals and subs are JL Audio E110’s. They re placed along the front wall, a quarter of the distance along that wall from the corners each.
Given the room size, one may think this is overkill (mains are full range) but small rooms are arguably more difficult to get good bass than large rooms. I started with one sub but bought into the arguments for two or more about six months ago. Because I use an integrated amp, I run the mains full range but have a high crossover point on the subs, 80Hz. I got to this point using a Umik-1 microphone from MiniDSP and Room EQ software. It is very interesting to take in-room measurements from the listening position and varying sub placement, crossover point, polarity, and sub volume. I had a significant null point at the listening position with the subs off. Turning them on and keeping the volume at reference level (zero), got rid of the null and smoothed out the bass response nicely. Like others, I find the subs help with bloom, even in the mid-range. Bass is nice and tight, with great attack and decay. The whole sound has a more satisfying roundedness without sacrificing detail.
For those of you with rooms that don’t create bass issues consider yourselves fortunate. However, for the rest of us (which I suspect is the majority) one well integrated sub will almost always bring a smile to your face and two will result in a Cheshire cat grin. Can the use of subs be “abused” in a system? For sure. There is always the temptation to “get your money’s worth” but the temptation must be resisted. In my instance, the volume is set at a point where there is no boominess but the overall musical presentation is more pleasing.