Subwoofers - crazy upgrade!

In my experience and according to the literature there is more advantage to dispersing the subs than stacking them. By having multiple subs at different locations you can compensate for room modes in a way that is not possible with stacked subs. This does require the use of processing such as Dirac or MSO, but it is really the only way to mitigate room modes at multiple listening positions. If you goal is only to optimize for the one sweet spot, however, that can be done with a single sub position....although still requiring DSP based EQ.

I have been using one subwoofer in my room for as long as I can remember until recently I got a pair of Velodyne SC-8. Also I have always used a powered subs with the most recent REL R-218 which got replaced with the Velodyne pair. I also found a Atlantic Technology SP-8000 earlier this year with REL and it was big difference.  Now with a Velodyne pair powered by a Crest Audio CA9, it is in different league. 

I have 2 subs one 15 and one 12. Both run summed mono. They sound best asymmetrical in the room with the 15 doing most of the work and the 12 mixing the nodes.

@audio_guy_uofw 

I am not aware of any literature on stacking subwoofers versus dispersing them but I agree with your understanding of the latter's advantage.  

The curve below shows how a four speaker distributed bass array (DBA) can not only extend frequency response, but perhaps more importantly, smooth it out. 

The two waterfall graphs below demonstrate the DBA's impact on room modes (standing waves). I believe the smoother frequency response and diminished "ringing"  are more important than the extended frequency response.

Quite simply, it works.

@audio_guy_uofw  I agree with your point. Stacking subs like the REL concept is good if the implication is to allow sound to more focal and directional which this curtails an "array" sort of speak. Logically, this will only work if you have a big enough room but it is not going correct acoustic room issues. Only way to correct  is room nodes in a fixed environment is either physically moving the speaker or seating location and treat the room or electronically apply processing which is more complex to achieve without the tools and resources.  

BTW, I still have my REL sub and I might not let it go anytimr soon but the Velodyne SC-8 pair just has more body and inpact then the REL but the REL is more musical. I like both REL and Velodyne along with SVS and Rythmik but they all have different philosophy and agenda. You need to find with one works for you.