Experience with stacked subwoofers?

Whenever you “stack” speakers of any sort you alter dispersion in the direction they are stacked.  Ever seen/heard of a sound “column”?  This is exactly the principle, stacking shortens vertical dispersion.  This is used effectively on tour sound system to shorten vertical output ( determining how far the sound is “thrown”, but maintaining a wide horizontal.  Stack the speakers side by side, same thing, shortens width and maintains vertical.  So in controlling bass in a space, stacking is a very common method that has been used by experts for many years. helpful in a very reverberant/reflective space.  It is not something REL developed.  Any competent speaker engineer would be aware of this principle.  This is the argument for and against these long tall or wide arrays of similar drivers, such as using multiple tweeters next to each other in one cabinet- it creates dispersion lobes that have to be managed;  its not hard to hear those lobes moving back and forth across the array.   Advanced DSP can alter this inherent principle but its not easy or simple or something you can do on your own at home.  

I've been part of designs in studios that use linear sub arrays to narrow the width of dispersion from the subs. 

Brad

@emergingsoul 

Stacking subs is not a gimmick or a marketing ploy.  But its effectiveness does relate to your goal sad others have said.  One sub is nearly Omni in a room and expose standing waves everywhere.  Moving it often just moves the stadium g wave/ null location.  If it’s for home cinema, most of the sub output is sound effects so almost everything relates to the screen.  If it’s for music, distributed arrays can work better using multiple sources turned down at lower levels  especially in ATMOS/ immersive.  I think Duke has spoken about this extensively. 
brad

@emergingsoul 

An awful lot of old people still voice “you shouldn’t need subs” and I think this was accurate in the days of Led Zeppelin, early Genesis, English rock and roll.  What I see these bands doing with subs on the recording side has certainly ended that idea. Pop music (Ariana and Sabrina carpenter ) - Americana ( Sarah Jaroscz) even country (post Malone/Jelly Roll/ chris Stapleton, Kane Brown ) -are working the subs hard.
 

Brad

Me too! 
brad