There are three reasons why rooms have so much more impact on the sound in the bass region than elsewhere in the room. The first is acoustic, and the second two are psychoacoustic.
Normal home listening room dimensions tend to be "small" relative to the wavelengths in the bass region, and so room interaction results in large peaks and dips, which are usually spaced fairly far apart.
The ear-brain system tends to average out the sound energy across ballpark one-third octave wide bands, such that narrow-band peaks and dips get psychoacoustically smoothed. But if the peaks and dips are large and far apart - as they typically are in the bass region - the ear's averaging mechanism doesn't help much.
Finally, the ear is literaly slow to register bass energy. It takes at least one full cycle for the ear/brain system to even register the presence of bass energy, and much more than that to discern pitch. So by the time the ear is detecting a bass note, the room's effect is in full swing. In other words, the ear cannot separate the room from the woofer in the bass region; they are a system.
Equalization/room correction systems can greatly improve the bass at a single location and the better systems use multiple microphone positions and can make an improvement over a reasonable area. But they are limited in their ability to effect an improvememt throughout the room because the room-interaction peak and dip pattern changes so drastically from one location to another within the room. Using EQ to tame a peak or fill a dip in one location will likely result in deepening a dip or heighteing a peak in another location.
There is an acoustic technique for smoothing the room-induce peak-and-dip pattern throughout the room. It calls for using multiple distributed bass sources; in other words, multiple subs (and they can be smaller since there are several of them). Each will produce a unique peak-and-dip pattern at any given listening location within the room, and the sum of these dissimilar peak-and-dip patterns will be significantly smoother than any one of them would have been. Any significant remaining trends in the bass region are more likely to be global (throughout the room) and thus more practical to address with equalization.
Multiple subs (even small ones) may not be practical in many situations, but in my opinion it's the front-runner if sound quality is the top priority. This opinion is shared by Todd Welti, Floyd Toole, and Earl Geddes (though there is some differene of opinion on the details).
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