Something that isn't obvious at first glance is that we literally cannot hear bass in our home listening rooms without hearing the room. The ear cannot detect the presence of bass energy without hearing at least one full cycle. And it cannot detect the pitch of a bass tone without hearing several cycles. Consider how long the wavelengths are in the bass region, and the size of our listening rooms, and you'll see that by the time you begin to hear a bass note, the room's signature is already all over it. So if high quality is the goal, we have to consider sub(s) + room = a system, because working on the sub(s) alone is ignoring the elephant in the room (which is the room itself).
A subwoofer may start out with a perfectly flat (and perfectly "fast") response, but by the time you hear it, the room has modified that flat response into a roller-coaster of major peaks and dips. Some subs will synergize better with a particular room acoustic situation than others, but no single sub can give you reasonably flat (+/- 3 dB) response anywhere in the room without equalization, and no equalized single sub can give you reasonably flat response across a broad listening area (because our room interaction peaks and dips change places as we move around the room, EQ that improves the response in one location is almost certainly making it worse somewhere else). So if you have a high quality speaker system, imo you're simply not going to be able to extend that quality down into the low bass region with a single sub, no matter how good it is. And having low bass that doesn't blend into the rest of the spectrum is often worse than having no low bass at all.
Nothing against bass traps, they can certainly help, but cannot correct the basic room acoustic situation of what the room is going to do to a single bass source. With multiple bass sources distributed around the room, they average out much smoother (and subjectively "faster") than any one alone. Intuition may tell you that the differing arrival times would cause smearing and blurring, but that intuition would be wrong, because the ear doesn't begin to have that kind of time-domain resolution at low frequencies. Remember our inability to even detect the presence of bass energy from less than one full cycle, and our need to hear multiple cycles to detect pitch? That's an indication of the ear's poor time-domain resolution at low frequencies.
I originally developed my multisub system (based on the ideas of Earl Geddes, which I'm using with his permission) to work well with planars. In the planar world, most who try a single sub go back to using no sub at all, because the discrepancy between what the sub is doing and what the panels are doing is just too great. On the other hand, most who try two subs end up keeping them, as they offer a worthwhile net improvement. Two subs are roughly twice as smooth in-room as one sub. Each time we double the number of subs, we essentially double the smoothness (or, halve the lumpiness), as long as our subs are spread around somewhat. This approach absolutely does not require special subs - you don't need to buy my system, you just need multiple decent subs. And they don't need to be large, high-output ubersubs because you're basically still getting ubersub air-moving capability by using multiple smaller subs.
Imo, ime, ymmv, etc.
Duke
