A normal home listening room will impose huge peaks and dips on the output of a subwoofer or pair of speakers.
As we increase the number of intelligently distributed bass sources in a room, we significantly reduce the magnitude of those peaks and dips. The only way multiple subs could fail to make the net response smoother would be if they all had identical in-room response, and the distribution ensures that nothing even close to that will happen.
So while adding a single subwoofer will offer the benefit of improved bass extension, adding multiple smaller subs and spreading them around intelligently will additionally result in smoother bass throughout the room. By way of anecdotal evidence, it seems like most Maggie and Quad owners who add a single sub go back to using their speakers without subs within two or three months. But it seems like most Maggie and Quad owners who add a pair of subs keep them in their system. Those few who have tried more than two subs never go back, as far as I know.
I have multiple customers who report +/- 3 dB in-room from 20 Hz up through the bass region without any EQ from using four small subs. And smooth bass is "fast" bass, subjectively speaking, because where there are peaks in the bass region, there is ringing. Smooth the peaks and the ringing has been addressed.
While the output of a single subwoofer can be equalized to be very smooth at a single location, or almost as smooth within a small area, such equalization will inevitably make the response worse elsewhere in the room. Because a distributed multisub system results in much less spatial variation within the room, any need for EQ is probably correcting a global (room-wide) problem rather than a local one. So EQ is arguably rendered even more effective by a distributed multisub system, assuming it’s even needed.
A distributed multisub system results in a greater number of smaller peaks and dips in the in-room response. The subjective improvement is often greater than the raw numbers would lead you to believe, and here’s why: The ear/brain system tends to "average out" peaks and dips that are within about 1/4 octave of one another, so when we have a lot of smaller peaks and dips bunched up together, they start to behave as a continuum (as far as the ear/brain system is concerned). This is what happens higher up in frequency (those hashy-looking room-reflection peaks and dips are there, but they’re so close together that we don’t hear them discretely), and this is also why large rooms have subjectively better bass than small rooms. A distributed multisub system can make a small room behave like a much larger room at low frequencies.
Here is what UCLA mathematics professor, concert violinist, and respected audio reviewer Robert E. Greene had to say about a distributed multisub system: "Makes even the most magnificent of one-point subwoofers into dinosaurs, something grandly impressive in their time, but their time is over."
So imo there is arguably significant room for improvement beyond the addition of a single good sub.
Duke
dealer/manufacturer
