Marty , I placed 1 in a corner, 1 behind me and 2 just plonked down where I had open space on sidewalls , 2 are elevated off the floor at differing heights , with the 4 subs and the 4 side firing 11" woofers of the speakers , the bass transducers are more or less randomly placed.
I use a Meridian 568.2 processor to set all 4 subs as to crossover , delays , level etc so can do all 4 simultaneously
1 sub is run out of phase.
I can turn all subs off with a button press or turn the mains off the same way.
The overall effect of using the subs is not really more or deeper bass, but much tighter articulate bass , and more weirdly a much better sense of the ambience of the recordings , a more spacious and defined soundstage..without them , the music tends to "collapse"
If you just play the subs , most folk are amazed at just how low a level they are playing at..the SVS sb13's are total overkill .. they maybe playing at 1/10th their ability .. The Yamahas would have done just fine.. but they dont go as low as the SVS's , and no where near as prestigious
Considering I dont low pass my mains , my notion is that bigger speakers benefit more with multiple subs when using the subs this way..
However I recently had some kef ls50's I used with 2 SVS subs where I did high pass the mains at 90hz and got excellent results..I would have kept the setup , but the ls50's just did not go loud enough for me and the room.
I have always used some form of DSP to integrate my subs.. mostly offboard stuff rather than the controls within the subs.
I think subs should at least be in pairs.. single subs seem to be a harder act...
I would post some pictures , but am in Ireland for a wedding , 6000 miles away from my system.