We have stacked pairs running in stereo of REL S812, Carbon Specials, and 212/SX. In each instance space constraints caused us to stack. When properly placed and dialed-in, stacked subs are terrific -- albeit I only have stacking experience with REL. And with REL's direct-to-amp wiring, I won't be using any other manufacturer's subs any time soon. Had other manufactures subs before REL, but there's no context. For not huge $$, Carbon Special has to be one of the best sounding subs out there, at any price point. (Note, I have not heard the Nos. 31 or 32).
@emergingsoul Sorry to pile on, but your take on subs is 15 years behind the times. Speakers in our main system are Tannoy Kingdom Royal, which include a 15-inch sub driver. Addition of a pair of Carbon Specials brought new dimension to the entire music spectrum. IMHO, no system, none, is complete without a pair of top quality subs. It's not just about the bass. Its about the entire presentation, from top to bottom.