Stacking subwoofers

Just to put things into perspective, I've owned dual JL Audio F110s with the CR1 crossover, dual JL Audio F112 v2, HSU, Martin Logan, dual REL Carbon Specials, REL G1 mk IIs and I currently use dual REL No. 32s subwoofers. I originally loved the REL Carbon Specials, but after a couple of months I was bothered by their down firing passive radiator and sold them. All rooms and systems are different, so the down firing passive radiator may not bother you. Based on my preferences and experience I would get a No. 31 or 32 in lieu of getting a stack of REL Carbon Specials. Of course it's just my opinion. 

Thank you all for your feedback!

FWIW one scenario I’ve found stacked subs to offer potential benefit (as opposed to the aforementioned careful placement throughout a listening area) is that in which sub(s) must handle higher frequencies ( > 80 Hz ) if the mains will not.

If that scenario is not yours, then stacking subs in a line array approach should only only offer the benefit of each sub having less volume (excursion) demanded of it - that could be helpful for a single sub that distorts on bass-heavy tracks.

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.