Stacking subwoofers

You would be better served by adding just 1 IF you can place it optimally so as to eliminate any existing dips.  If you can't choose where to put it bass traps and EQ may be a better direction for you at this point.

Seems to me that one would be better served by spreading additional subs around your space, rather than making a stack. Adding more subs in the same spot does nothing to provide additional mitigation of any peaks and valleys in the rooms low frequency quality.

Sub stacking is a bit of a goofy phenomenon observed in some videos these days. Don’t get inspired by such videos and waste too much cash in that manner.

Depending on your room, setting up a single sub near-field (you may even be able to set your whisky glass on it) and some positional eq could nullify the need for additional subs (at times), i.e., when you are optimizing for a solitary listening spot.

You could also get discrete in-wall/ in-ceiling subs at the right locations. But, modifications to some simple dry wall and in-wall wiring can be extremely scary (terrifying) for some audiophiles.

Since both your speakers and your Bass Speaker are -6dB sub bass systems their low frequency roll off is so dramatic its likely neither will excite your rooms standing wave bass modes regardless of where they're located. This opens the door for almost limitless positioning scenarios.

Regardless of how many or which models you choose the delicate sub sonic low frequency found more and more in todays recordings will be rolled off somewhere in the thirties. Those six packs certainly look menacing don't they? 

I had a 9K REL Studio III "SUB-BASS SYSTEM" in my house for two days. Beautifully made British cabinetry. 

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.

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.

I still remember the conversation I had with Randy from JL Audio. I mentioned that I had 4 JL-F113 in the corners of my room and he suggested stacking two of them in the front corners instead. He said that the Gotham series is very similar to that approach. Never tried it but, I still wonder...

ozzy

To conclude: I didn't go for three REL Carbon Special's stacked. It would also require additional connectors, more power cables, a tower of speakers in a living room with cats, etc.

I eventually bought a REL No 32. Works just fine :)