That's a great article and confirms what I stated. Placement of the "Sub" can be critical. Not the mains. Being involved with PA (somewhat) I have been in discussions of low frequency cancellation. That can happen with 2 subs and is a good reason NOT to have a system set to "large" where the mains are carrying the same frequencies as the sub. It is the very reason why you should be setting your system to "small" when using a sub.
A question that constantly comes up on the PA form I visit is when using a double 15" with a horn cab. The bottom 15" is considered a "quasi sub" that extends lower than the upper woofer yet no where what a real sub will produce. Now these folks want to place a sub underneath this speaker to get real bottom. There is way too much overlapping frequencies with this arrangement and usually gives a muddy low end rather than the big bottom that they are expecting using "extra bottom". This also goes along with the "small" settings in HT. There is also complaints of phase cancelling where ther is loss of bottom. But this cancelling happens more with 2 subs than with anything else.
If you had your system properly set-up with the small setting than you should have heard your sub no matter where it was placed and it should have been loud. My understanding of sub plcement in HT is more for smoothness and level frequency throughout the room (boominess). I still can't see the mains placement having anything to do with the loudness of the sub.
But I really am not that knowledgeable in any of this.
