"While a room resonance produces an amplitude boost at one point it
can be contrastingly quiet some inches away, and the response also
depends on the position of the sound source."
So when one has several sound sources at different places, the effect
of positioning is averaged. In the bass range where one cannot detect
the direction from which the sound is coming anyway, this is a viable
solution. It is also a good solution to have much diaphragm area (or
mouth area in case of horn speakers) and aligned in the right manner,
because of directivity which can defeat resonances as well.
When you experience more loudness in case that two speakers are
located close together than in the case that they are far away from
each other, that is true (because of radiation impedance). But sound
quality is not the same as loudness.
Cut short: There are many more things about acoustics in the world
than loudspeaker sales people tell use.