Stacking subwoofers

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.