UPDATE - This may be some explanation for the discrepancy in opinions here. Last night I experimented a bit with a 40 hz test tone to see if I could hear where it was coming from. This wasn't scientific since I knew where it was, but I'm pretty sure at LOW and MODERATE volume I couldn't "hear" it's location.
HOWEVER - above a certain fairly loud volume point, I think I could FEEL where it came from, literally - through my clothes. This makes sense (I think) - the primary pressure impulses are hitting your body from a specific direction, and you pick it up more on that side than on the other, just like wind blowing at you.
And the volume is important - just as it's much harder to tell the direction that a light breeze is coming from than a stiff wind is hitting you from.
It's possible that your brain can integrate the directional information felt by your overall body with the non-directional ear input, so that you think you are "hearing" it from there - or in essence ARE hearing it.
I then played some LOUD rock/blues with electric bass, probably centered around 65 hz or so, which I remembered being way over to the left side. I sent it through my main speakers ONLY and concentrated on what was happening. (CD was Joe Louis Walker - Great Guitars)
I could "tell" (not necessarily "hear") that the bass was from the left, and I'd guess it was a combination of feeling the impulses bodily and the higher overtone cues. But the next step would be to try it with the single sub crossed over high enough to really exclude the mains from the lower frequencies and see if the bass "imaging" suffered. Unfortunately the sub now has a custom fixed crossover point mated to my speakers and I can't do that easily :(
In addition, I'm giving the sub away to my cousin this weekend and won't have one for awhile. WAF issues - my wife wants me to get one (or two) of the smaller long-throw cube types.
But my guess is that both sides of this argument have merit depending on SPL and source material:
--At high enough SPL the brain may incorporate information that DOES NOT COME FROM THE EARS, and insert it into the brain's locatization algorithm. I'm not sure if this is anything normally considered in acoustic theory.
--In addition, of course, the source material actually has to have stereo bass - which as we've discussed before is virtually non-existent on vinyl lp's. So for vinyl people like myself, two subs is a total waste. For CD it may be desirable depending on how loud you listen and how much source material has the true bass frequencies actually mixed to one side. And for DVD it probably should be a good idea.
