Hello cleeds,
We all are unable to localize deep bass frequency soundwaves, that is determine where the sound is coming from, that are below about 100 Hz but we're very good at localizing higher frequency soundwaves in the remainder of the audible spectrum, from about 100 to 20,000 Hz.
This is the reason there's no such thing as 'true stereo' deep bass and why the bass is summed to mono on frequencies below 100 Hz on all vinyl and cd recordings. If you doubt this, try to find a single vinyl or cd recording that is not summed to mono. This means it's pointless to configure subs in a stereo configuration with one located by the left main speaker and one by the right.
However, thanks to psychoacoustics and our remarkable brains, it is possible to create the perception of stereo bass in our systems. Here's how it works:
Whether you use 2, 3 or 4 subs, run them in mono and optimize the bass at your listening seat. The bass below 100 Hz won't be able to be localized but there are bass harmonics or overtones of the deep bass fundamental frequency that extend into higher frequencies that are reproduced by the main stereo speakers and can be localized. Our brains are able to associate the fundamental deep bass frequency reproduced by the subs, that are not able to be localized, with the deep bass's higher harmonic frequencies, that extend well beyond 100 Hz, which are reproduced by the main speakers that are able to be localized. This psychoacoustic association allows us to localize the deep bass in the soundstage, for example the kick drum is located in the rear center and the upright bass is located in the front to the left, which would not be otherwise possible without this psychoacoustic association our brain's are capable of.
