JJ covered this over at AA, so i'll try to summarize what the thread entailed. Even though the output of a sub is centered well below 100 Hz, most of what we are able to localize is the actual distortion coming out of the driver. Since the harmonics are above the primary frequency of operation, you can still form a sense of localization. As such, going to stereo subs tends to stabilize the image and even out the presentation across the soundstage instead of having the sound lumped or confined to one specific area.
A point that i brought up in that discussion is that downloading seems to mask the ability to localize a subwoofer. JJ's rebuttal was that a downloaded driver will have self cancellation of the higher frequency harmonics due to reflections in the pressure zone of the driver. Further reduction of harmonics is also heightened by the ( typical ) placement of the subwoofer on carpet and padding.
Speaking from experience though, downloaded subs work best only for extreme low frequency operation ( like what SUBwoofers were originally intended for ). They lack any type of punch or definition for upper bass notes due to the lack of direct radiation. This is also the main reason that you don't hear the distortion out of them, as it is directed away from your ears.
Downloading is also VERY tough on a driver, especially those designed with a lot of mass ( this lowers resonant frequency ). The weight of the cone tends to pull the voice coil out of the gap and sag the suspension. As such, excursion is minimized or at least drastically reduced. For that matter, so is power handling. A woofer that could normally take a "gazillion" watt pounding may now only handle a few hundred watts. AS such, a driver used in a downloaded sub needs a REALLY stiff suspension in order to operate best while maintaining good linearity and power handling. I found this out the hard way AFTER building a few downloaded models and then "throttling" them to death. Sean