Stacking subwoofers

In my case it worked!
I had a problem with bass response in my old living room with low ceilings, 7’8” with hard “plastered ceilings”.

Had dual (stereo) subs at the time, with lots of bass traps. My impression (and others) was while standing and/or mulling around listening, the bass response was nice and tight (punchy). But once you sat down, that sensation was gone. We presumed it was a standing wave problem, due to the shortest (and usually most offending) dimension of the room (floor to ceiling)

We tried setting subwoofers at mid height which did help, but it wasn’t until we set up a single bass array (sort of a stacked sub or array approach) that it eliminated the problem. 

Most folks aren’t willing to do what it takes with multiple subs due to finances, aesthetics or both, so they compromise (live with bass nulls and or just sit (or stand!) in the best spot).

For me It’s redundant and obvious to say that if your insisting on uniform and even bass response throughout your room, then the subwoofers orientation must slave to its dimensions.

One of the things that I don't understand is that a lot are using a "pair" of subs. I was taught that bass is non directional. I am also a firm believer in crossing over between 80-90Hz. I find that going up over that allows voice into the sub, and that seems to be where problems begin. I've always used a single sub. Not saying it is right or wrong, just find my system to sound great and the sub blends in well.

In regards to putting multiple subs around the room (rather than stacking them), I believe if you could do a 3d graph of the listing area, it would be like a mountain range with peaks and valleys all over the pace.

I too was taught base is non directional.  If you were to place a sub next to each of your speakers, using the same front plane (minimizing out of phase issues), you will notice base is very directional and melds well with your existing speaker(s).  There is no comparison to a single.  The fact that you are separating the L and R base also prevents your system from playing multiple base notes from guitar and drum that are often very separate but, fight each other when combined in a single sub.   If that is not enough base, add more or buy bigger.  JL Audio shows placement up to 6 subs in a room.

In my HT room, my subs were placed right besides the L/R speakers. It made placement of the L/R speakers hard, as it was taking up the entire wall, so I stacked both subs in the corner. To my surprise, the bass opened up, and was deeper. I had to turn both sun down. Not sure if it was the stacking, or placing them in the corner. For my room it works. As a bonus my L/R speakers are in a much better position. 

Yes, bass is suppose to not be directional, and it moves faster through a solid object. Starting to think all the rules are changing with digital, and room correction. My 2ch system follows all the old rules and that works for it. 

Why vertical arrays instead of horizontal arrays?  The answer is probably in the room mode you are having problems with.  I encourage the use of the AM Acoustics room mode simulator to help understand your room modes as well as opportunities for speaker and bass trap placement.