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.