Subwoofers - crazy upgrade!

@gdaddy1, I've not got any comment on 6 db vs 3 db gain but one thing is clear for multi subs improving bass.  Multiple subs create multiple room node patterns.  The idea is to have "more" of them but in differing locations.  This will reduce the extremity of the summed nodes at any one location.  Less "sweet spot" and "suck out" locations.  Fidelity of each sub notwithstanding. There have been many threads based on DBA and this topic.  @deep_333 that looks awesome.  But since room nodes occur at even fractions of room dimensions (1, 1/2, 1/4) it might be optimal to have 2 of the 4 at odd ratios like 1/3 of the wall lengths.  But if one has a dedicated space and sitting location (most here do that), maybe this isnt a worthy consideration. That was my takeaway from my reading and understanding of the virtues of multi subs.  Personally haven't gotten past 2 subs, lol.

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.