[Charles]

"By carefully locating the loudspeaker and
listener within the room the frequency response can be
made relatively smooth at a single listening location, as in
the following example."

I think the key is how many people and listening locations? I listen almost exclusively by myself. I have found that a single very large very high quality sub located in the plane of the main speakers and exactly between the main speakers can work really well and this sentence seems to support my findings. Multiple subs require multiple amps, wall sockets, cables, cords, etc. The typical setup you see is stereo subs in corners well behind the main speakers. I have never understood this rationale. In practical terms stereo subs is about all you see because of the total impracticality of 4 subs. I don't think stereo subs in corners smooths out anything. As far as dynamic range 2 subs only provides an extra 3 dB, totally inconsequential.

Charles