Stacking subwoofers

I too was taught base is non directional.  If you were to place a sub next to each of your speakers, using the same front plane (minimizing out of phase issues), you will notice base is very directional and melds well with your existing speaker(s).  There is no comparison to a single.  The fact that you are separating the L and R base also prevents your system from playing multiple base notes from guitar and drum that are often very separate but, fight each other when combined in a single sub.   If that is not enough base, add more or buy bigger.  JL Audio shows placement up to 6 subs in a room.

In my HT room, my subs were placed right besides the L/R speakers. It made placement of the L/R speakers hard, as it was taking up the entire wall, so I stacked both subs in the corner. To my surprise, the bass opened up, and was deeper. I had to turn both sun down. Not sure if it was the stacking, or placing them in the corner. For my room it works. As a bonus my L/R speakers are in a much better position. 

Yes, bass is suppose to not be directional, and it moves faster through a solid object. Starting to think all the rules are changing with digital, and room correction. My 2ch system follows all the old rules and that works for it. 

Why vertical arrays instead of horizontal arrays?  The answer is probably in the room mode you are having problems with.  I encourage the use of the AM Acoustics room mode simulator to help understand your room modes as well as opportunities for speaker and bass trap placement.

I still remember the conversation I had with Randy from JL Audio. I mentioned that I had 4 JL-F113 in the corners of my room and he suggested stacking two of them in the front corners instead. He said that the Gotham series is very similar to that approach. Never tried it but, I still wonder...

ozzy

To conclude: I didn't go for three REL Carbon Special's stacked. It would also require additional connectors, more power cables, a tower of speakers in a living room with cats, etc.

I eventually bought a REL No 32. Works just fine :)