According to Earl Geddes, whose ideas the Swarm is based on, the in-room smoothness goes up as the number of independent bass sources goes up. So four subs intelligently-distributed are theoretically twice as smooth as two (and "smooth bass" = "fast bass" because it is the in-room peaks which take longer to decay into inaudibility). In practice the measured improvement is typically not quite as good as in theory, but is still significant.
A little bit of frequency response improvement in the bass region goes a long way, and this is because the ear is actually more sensitive to changes in SPL at low frequencies than higher up the spectrum. If you eyeball a set of equal-loudness curves, you will see that they bunch up in the bass region. So a 5 dB change in SPL at 40 Hz is subjectively comparable to a 10 dB change in SPL at 1 kHz! This heightened sensitivity to SPLdifferences at low frequencies is one of the reasons why it often takes a long time and a lot of fine-tuning to "dial in" the ideal gain setting on a subwoofer amp.
Also, it is not necessary that all four subs be identical. In fact in his own distributed multi-sub system, only one of Earl Geddes' subs extends all the way down.
Duke
Swarm manufacturer
