Exactly, depends on room dimensions.
For example, in your 11-foot wide room, your first 4 width modes will be around 51Hz, 103Hz, 154Hz and 205Hz.
If you could visually see the behaviour of the modes across the width of your room, this is what they would look like:
So if you played a test tone at your 1st width mode (51Hz) and walked across the width of your room, the sound would be loud at the left wall, become very quiet at 50% (midpoint) of room width, and become loud again at the opposite wall. (see first graph in diagram above)
When playing sounds that fall at your second width mode (103Hz), it would be loud on your left wall, become very quiet at 25% of room width, get loud again at 50% of room width, become quiet again at the 75% mark, and loud again at the opposite wall. (see second graph in diagram above)
Of course you're not going to be walking across your room when watching a movie, but if those peak & null points fall at/near seating locations, that is a problem. Imagine the person at the middle of your sofa hearing certain frequencies very quietly while the listeners on either side of him are complaining that those same sounds are too loud.
Placing your source of sound pressure (subwoofer) where sound pressure is lowest (null) will cancel that mode. Toole's example above can cancel the first 3 width modes of a room. But that is not useful in your room because your subwoofer won't be playing at 103Hz & 154Hz (your 2nd & 3rd width modes), since it will be crossed over lower (80Hz?). If the subs aren't playing sound at those frequencies, how can they have any effect (let alone cancel modes) at those frequencies.
However, your sub will be reproducing sound at 51Hz, so you can use it to cancel your 1st width mode by merely placing it at the null of that mode (hence my suggestion in your other thread). To get the same result using absorption would require very thick bass traps.