Experience with stacked subwoofers?

Have been using stacked subs in my HT room for almost a year now. Still sorting the room out, and don't have long enough cables to put them in the proper place. 

To sort out L/C/R speakers, the subs had to move, so they go stacked in the corner of the room. Yes, ran DSP after. Both subs have footers under them, to isolate from each other. 

They sound just fine, get all the bass I could ever want and then some. Once stacked, both needed the gain to be turned down. Think it's a little cleaner, but these are movie subs, not musical. 

@bobelton What is your low frequency goal?

1. The addition of the subtle extra low frequency from instruments and effects found on so many modern recordings? 

2. Reinforcement of your current speakers woofers?

Speaker positions are often in an area of the rooms null. A null can deplete the bass output of most any speaker even those equipped with powered subwoofers.

This from a brochure ["REL subwoofers (I still like our original term Sub Bass System for its more complete description of what we do)’]

Since -6dB Sub-Bass Systems (woofers) roll off so dramatically they simply do not excite a rooms standing wave bass modes which allows them to be positioned most anywhere in a given room. The trade off is the rated extra low frequency rolled off output is barely audible regardless of how many woofers are used. 

Despite this limitation and for what ever there experience, there are many satisfied users.

If your goal is number 1 you’ll need to be shopping for actual -3dB subwoofers.

Here a subwoofer array (distributed bass array) is typically corner placement of four cabinets were their output loads the room equally mostly eliminating the room standing waves. The extra low frequency presentation is quite stunning in my in home experience.

Using one to three -3dB subwoofers requires more precise positioning.  All the best in your search. 

I've seen a Youtube video on this very subject. I found it when I was researching Rythmik Audio subwoofers.

If you have decent speakers you only need one subwoofer maybe two.

It’s a gimmick from a marketing standpoint. Not sure how meaningful the changes and I’m not hearing much on what it does up above and there seems to be limited enthusiasm.

Why not Stack tweeters and mid range drivers.

I stack my dishes and silverware and that really works good.

A Ripole Subwoofer might be the one to investigate.