The real answer is: It depends......on the cables used. and the type of polished rocks used to calm the eddy currents therein. So all of you non-believers need to be enlightened. Come drink the Kool-Aid.....
OK, seriously. In a rectangular room, with identical walls, with the Schroeder frequency above 150Hz or so, with a central seating configuration, in order to get the most equal (not flat, mind you) frequency response between seats, yes, more than 4 LF source locations of emenation are not necessary.
PLEASE READ THAT LAST PARAGRAPH AGAIN.
But most folks do not have this ideal room, and experimentation (and delay and PEQ, if you have that capability) and measurement (and more measurement) are key. To save money, I would not recommend using more than 4 sub locations
, as it would not necessarily yield greater uniformity between seats.
In larger rooms where Schroeder frequency is much lower, all bets are off on multiple LF source locations. This is why big theaters cluster their subs, for more output due to mutual coupling:
Spreading out subs throughout a room lowers overall
LF output compared to clustering subs, until you get to the PVG region, where every new sub adds to the pot.
Wanna throw another wrench into the worx? Measure your room's response at your calves vs your abdomen vs your head. Find out that the 'pant-flapping' and 'gut churning' you were 'hearing' was anything but a flat freq response.....and have your mind blown.....
But seriously, get the good cables.