Bass response - how deep should my speakers go for a lifelike sound at home?

[Antoine]

Subwoofers are not just about low frequency extension. Subwoofers allow one to even the bass response in a room without compromising optimal main loudspeaker setup. The best spot for a speaker in regard to (for example and not limited to) stereo imaging is hardly ever the best spot for the best bass response.

Low frequencies cause the biggest acoustic troubles (and the hardest to "fix") in most (all!?) rooms but the good thing about them is that below a certain frequency treshold (some say 80Hz, some a bit more or less) the human hearing can't locate the exact origin of low frequencies which makes it possible to place one or more subs and place these at the best spots in a room for LF reproduction while at the same time optimal main LS placement after off loading LF reproduction from these mains.

After having owned multiple full range loudspeakers I today would probably buy smaller loudspeakers with less LF extension and augment them with multiple subwoofers. And these would not all need to go down to the lowest sub bass frequencies!

I think that most people with average musical preferences and average rooms after having heard a proper sub, properly setup so that it blends in well, would prefer to not go back to a regular setup without a sub. (a/one sub can of course be substituted for multiple subs in this sentence).

You are quite right about the bass in room, of course.
Personally I think a single sub is hopeless though, it is impossible to position for good bass in a large part of the room, and if using calculated correction one can only get it reasonably right in one part of the room, and it is very often worse everywhere else.
I tend to agree that having smaller main speakers and 4 subwoofers would probably give the best overall bass even-ness in room, but so far I have heard very, very few speakers or subwoofers which have realistic sounding bass, whether even or not. When I spent a couple of years 20 years ago auditioning speakers with an almost limitless budget I found that realistic bass could be reproduced (I had previously accepted that it was impossible domestically) but very, very few of these very expensive speakers did it.
Plenty of extension was available with all of them at this price, but realistic?, almost none.

I'm not sure if one single sub in a room paired with DSP (especially one that doesn't try to boost dips/nulls) is any or much worse in bass evenness than a pair of speakers. I guess that depends on the setup of both, but of course more LF sources would be better which was exactly one of my points as well.

Personally I set up my system for best sound at a certain sweet spot and don't really care about the rest of the room.