It is actually totally possible to get flat bass response in more than one seat with multiple subwoofers and EQ to take the room modes.
So why do we still need to have acoustic treatment and bulky bass traps? Grimani said it best - the room is the Boss and will impart it’s sound and make all the expensive hardware perform less than optimal.
Even after flattening all the modal peaks, there is still strong possibility that we hear the room more than we hear the content of the bass/movie soundtrack.
The reason is decay time. Especially in small rooms, multiple subs required to achieve even bass response really overloads the room with sound and bass energy… Without any treatment, the energy hangs in the room long after the signal has changed or decreased below the room energy level. When that happens, we are hearing the reverberation of the room, not the sound being playback.
We are all roughly familiar with the concept of room modes, with the first modal peak related to the longest dimension of the room. A 4m length with have the first peak ~ 42Hz.
There is also a zeroth room mode - below the first modal peak. The entire room is pressurised, imagine trying to blow up a balloon inside a box. The balloon will press against the walls of the box. That’s what the zeroth mode is doing. Any bass below the first mode is literally trying to pressurise the room.
All these bass energy has to go somewhere, or it hangs around a long time and the bass sounds slow and muddy and less defined.
This is the reason bass traps drastically improves the quality of the bass sounds in the room. It will not actually have much effect on a modal peak, especially in the 40-50hz range. Those still need to be Eq-ed out. Even 8-12 large bass traps in a room setup will only reduce the main peak by ~5-10db at best. And peaks can easily be 20db in a bare room.
What the bass trap does is to dissipate the sound energy, so that we can hear the speakers and subs playing the soundtrack, not the energy overhang in the room.
This is the way.