Experience with stacked subwoofers?

@lonemountain  I spend the afternoon setting up the first four REL S/510 of my six-pack line array....and damn does that sound good...and I still have some fine tweaking to work on.:)

And I am an "old people." 

After the ten minutes spent locating my rooms two strongest standing waves I position the two twelve year old subwoofers within. The twenty minute Auto EQ runs while a Sweep Tone CD is played through the main speakers as each subs fourteen parameters are adjusted to both the speakers and the room simultaneously. Followed by a minor five minute manual tweak to personal taste all from the listening position, close the laptop and done. 

I'm not only old, I'm a lazy idiot. I still have that silly Columbia 'Sneakers' LP - that 'Cosmo' (Kramer) track, jeez. Best of luck with that. 

I added a six pack of REL Carbon Specials to my Rockport Cygnus (which have great bass already for size and price) about 6 months ago. The added bass support was fun with modern soundtracks, but the creation of the "space" of the venue was something I have never been able to access before. My wife thought here he goes again on a wild goose chase, but was immediately saying that she loves them. It is the most significant upgrade beyond different main speakers that I have achieved. Once you hear what they can do, I don't think anyone will want to go without them. It's so not about more bass, the first two subs did that. It's about how much more engaging the experience becomes. If you notice several top of the line exotic brands have 4 box speakers with two of the boxes containing...you guessed it, three woofers. This set up is similar to my six pack next to my Rockports. Everyone who can try a 6 pack of subs should try it. I can't imagine going back to just mains.

@wokeuptobose 

I have been dialing in my second pair of REL S/510 as a four-pack array, while I wait for delivery of the third pair. Even going from a stereo pair to a four-pack has been an amazing revelation. My wife was skeptical, but she is enjoying the change immensely and starting to ask me to play stuff she loves to see how much better it sounds. She's now asking when the last pair will arrive and get integrated.

I should get my third pair of S/510 soon to complete the six-pack. This is becoming a revolutionary change to my system. MIND. BLOWN.

@wokeuptobose & @dwette 

YES, Yes, yes.  This is what I heard at the AXPONA demo.  While the six-pack clearly delivers more bass (yeah, they're subwoofers, so that's obvious) the impact of the RELs was shockingly apparent with regard to: (1) size of the soundstage (2) precision of imaging.

I went into the demo thinking this was just RELs attempt to sell more subs (a "marketing gimmick" as @emergingsoul put it) or more charitably an attempt to even out bass performance (as multiple members have implied with their comments).  I left the room shocked and awed. My first thought was... how can I ever again assess the sound quality of ANY pair of speakers without an accompanying REL six-pack?  I'm sure many will write me off as a kook.  That's fine with me.  I heard what I heard. 

Which manufacturers other than Rel make subwoofers specifically designed to be stacked?  I’m assuming here that there are different design decisions that a manufacturer would need to make for their subs to be optimized for stacking.  In particular, are there any stackable subs below the Rel 510 price point?