After reading much of this debate, it is good to see that Raul & Truthseeker agree. Both agree when using low distortion subs at low frequencies (less than 40hz or so) that one cannot determine the location where those very low notes emanate from.
At these low frequencies the wavelenghs are so large, relative to the distance between the ears, that the source cannot be localized. This is because wavelength is inversely proportional to frequency (thus reducing the frequency by 50%, as from 40 hz to 20 hz, the wavelength is doubled in size).
Using a large driver with very low distortion (such as the Velodyne DD-15 or DD-18, rated at < 0.5% distortion), with a relatively low crossover point, so that the sub puts out very little information above 80 hz (relative to the mains), which is located in a time-coherent location (with correct phase, of course), when placed in a room (as opposed to placement outdoors suspended above the ground) the spatial cues for locating deep low frequencey notes are lacking, so the speaker's location cannot be detected.
If the crossover point is set so high as to allow upper bass (and higher) frequencies to be audible, then spatial cues become increasingly present and identifiable. Smaller drivers and higher crossover frequencies result in the intrusion of subwoofers into regions where they should not intrude.
However, if the equipment and room requires a setup where subwoofers operate at upper bass and higher, then they will be locatable by ear. Under these circumstances, although the location of the low bass notes cannot be localized, their higher level harmonics (and other spuriae) will be audible, thus allowing a degree of localization to be perceived. This is not ideal.
Ideally, the mains will create the soundstage - not the subwoofers. One or more very high quality subs with low crossover points, which are ideally matched to both the main speakers and the room, are the best way to go. I hope we can all agree on this last sentence.
By the way, I am not sure how much phase and time coherence affect this, but it seems logical to always strive for the correct phase and the most time coherent location feasible in a given room with any given system.
I really enjoyed reading the debate. Every opinion is welcome. Personal attacks are not.
What I would like to know is, assuming cost is no object, how many subs are best and how should they be placed in relation to each other?