Two vs four subwoofers for music playback

At least one of your subs should be a Velodyne dd+ so you can dial in perfect bass response at your main listening position at all levels. Preferably a 15".  :-)

If 2 is good, 4 must be better. Hell,12 should be the gateway to enlightenment!!!!

@aldnorab - I have heard of their being used in either way (stereo or mono). In my system, I use the 3 in mono, because of equipment limitations. Also, it’s hard to run 3 of anything in stereo! I ran the 2 in stereo before that.

As to spaciousness, that’s an interesting question. I am not sure how much spaciousness in stereo subs is real (i.e., on the recording) and how much is caused by slight phase differences, which might create a sense of spaciousness. IMO, it’s one of those areas in audio where it’s difficult to know exactly what’s going on. If I had 4 identical subs, though, I think I’d try them in stereo. One reads that many modern recordings do have stereo bass, and it would be interesting to compare.

My main speakers already go to 20Hz. But due to a standing wave in the room, there was no bass in the listening position! I added a pair of Swarm subs and it took no fiddling (once I placed the two subs I've not had to move them at all nor mess with their wiring). I set up the subwoofer amp per Duke's instruction and they worked a treat. I can tune the sub amp on and off and the difference at the listening position is profound. Its worth noting that nearly everywhere else in the room there is bass without the subs. I amp limited by the room layout as to where the listening position is though. So this proved an elegant solution.