Main systems/Main room (Giya or Focal) - 1 or 2 SB16 Ultra - not really required in general, but certainly adds a lot to some stuff (organ music being demo stuff, but quite a bit of modern stuff as well). REW/Umik optimized
Home Theater 5.2 system usually - 2x SB2000 (+ 1 SB16 Ultra depending on the mood). Mostly 2x SB2000 because that's what the AVR supports. I let Audissey do its thing, relatively happy with it, but I do not use that room for music as I don't really like the Klipsch Reference I have there for that use (in stereo). The pentagonal room seems to help for multiple listeners.
Office - LS50 + 1x SB16 Ultra - REW/Umik optimized. A huge gain in bass obviously. Had to tweak the SB Ultra internal EQ a lot to get the response I wanted. I have two FR profiles, one is the "gold standard"
and the other one is a bass-boosted one which I use for different types of music.
Gym - 7.1 with one cheap yamaha sub and seven small monitors. Initially, the gym had no sound system, but over the years, the stuff I did not use anymore found a second life there - it isn't intended as an audiophile setup, but the old not really good yamaha sub added a lot
Main systems: could live without, a nice plus in some cases. Home theater: mandatory. Office: not mandatory, but really really great. Does not fully bring to the level of the big systems, especially when listening carefully, but first impression is similar.
General thoughts...
- lots of sub crawling to find the best spots.
- default settings, manual tweaking were hopeless in my case. At least in my experience, you can't expect to drop a subwoofer and maybe toy with the phase/volume and get good results. SB16 Ultra in the office (approx 4.5m by 9m) is usually working at -23db + DSP tweaks to get "gold standard" response.
- when the subs are properly integrated, it just disappears. Adds to the experience, but don't attract attention to themselves.
- somewhat unexpectedly, I also found a spectacular benefit of their use in some natural, live acoustic recording where I could hear the musicians moving around the scene, their footsteps echoing on a wooden scene, etc... It isn't really about music, just the atmosphere, the illusion of being there.
And, of course, let's not forget the best subwoofer accessory you can buy