The elephant in the room IS the room. The room’s effects at low frequencies are huge. Like enginedr1960 above, I’m a disciple of Earl Geddes when it comes to how to do a subwoofer system right. The justification for using multiple (often four) intelligently-distributed smaller subs is qualitative, not quantitative: The sum of their multiple dissimilar in-room curves is quite a bit smoother than any one alone. And smooth bass is "fast" bass because the peaks are where the energy takes longer to decay.
Most speaker/room combinations have significant room for improvement in the bottom couple of octaves, so subwoofers done right can make a worthwhile difference.
Note that the ear is actually more sensitive to differences in SPL at low frequencies than it is higher up the spectrum. This is revealed by the way equal-loudness curves bunch up south of 100 Hz. In the 40 Hz region, a 3 dB change in loudness is subjectively as large as a 6 dB change in loudness at 1 kHz. That’s why bass peaks stick out like sore thumbs.
As a general principle, the in-room smoothness increases in proportion to the number of intelligently distributed bass sources. So two subs are potentially twice as smooth as one, and four subs are potentially twice as smooth as two (and that potential is not elusive - but neither is it automatic). And unlike smoothness achieved by EQ alone, which is limited to a small listening area while the bass is actually made worse elsewhere, the smoothness of a distributed multisub system holds up well pretty much throughout the room.
Duke
dealer/manufacturer/Geddes Mafia charter member