I bought 4 subwoofers and I'm absolutely not doing a DBA! Hah!

Sometimes its really hard to bring people along on a fun passion project. 🤣

@erik_squires I know exactly what you mean. Everyone has different ideas and I've been told many times mine won't work. Or asked why I'm bothering and so on. But passion for the sport is passion and if we don't have that and a vision for the project then IME there's no joy in proceeding.

So while we might not agree on exactly how to proceed that doesn't mean I don't support you in this. Hobbies are meant to be fun!

@devinplombier 

Are your cabinets partitioned horizontally so that each driver has its own sealed enclosure? It’s hard to tell from the drawing you posted.

No, the horizontal braces are well ventilated.  Since the drivers are identical AND they will have the same electrical signal there is no need to isolate the cabinet spaces.  Also solid bracing reduces the internal volume so would require a larger cabinet. 

See Troels Gravesen’s site for other ways of using ventilated bracing, and for fun look up the B&W Matrix designs. 

Also, what is the rationale for the recessed amp housing compartment 

Aesthetics.  This is how they were designed to be installed. .  See the back of any powered subwoofer ever made.

and wouldn’t it be preferable, generally speaking, to leave the inside back panel of the enclosure flat?

Not really.  In truth many have suggested that turning the insides of a speaker into a kind of diffuse field that avoids coherent (i.e. flat) reflections is the way to go.  At the wavelengths involved here ~ 7' I'm not concerned.

That could be done simply by knocking out the amp housing, which would also provide opportunities to implement better ventilation for your amps, which will need a lot of it

The enclosure for the amp is really to reduce the opportunity for microphonics and long term part stability.  The enclosure is, again, as recommended by the manufacturer.  Alternatively you can avoid using the enclosure altogether if you can guarantee a minimum of spacing between them and anything flammable.

@clio09 

I have 4 of the RSS265HF-8 woofers sitting around doing nothing that I was going to sell. Now you have given me something to think about as I need to downsize as well.

I’m glad I could tickle your own interest.  While I did opt for 2 cabinets you could also consider using only 1 woofer per cabinet and 2-way amps instead of three, perhaps giving you the ability to put a cabinet under each surround speaker as well, if you were so inclined.   The drivers however are not the most expensive part of these builds.  The cabinet and amps are.

@clio09 wrote:  "I recall hearing that system in Long Beach and liked the concept. I might like to try something like that. If you don’t mind sharing, what were the dimensions of the cabinet that was used in the set-up?"

The Long Beach version of the "SuperStand" was about 24" tall by 12" wide by 15" deep, base included in the height dimension but not in the others. The footprint of the base was 18" square.  Some of the internal volume was devoted to an up-and-rear firing 10" coaxial driver, and some of the internal volume implied by the outer dimensions was lost to a large "notch" in the back, with the woofer mounted on the upper surface of the notch and the coaxial mounted on the lower surface of the notch.  The woofer was in a sealed enclosure.

The speaker atop the SuperStand was 23" tall by 12" wide by 12" deep.

Link with photos in case anyone wants to see what we’re talking about; scroll down a bit for photo showing the notch:

New gear from AudioKinesis and Resonessence Labs | T.H.E. SHOW 2019 - pt.AUDIO

Duke

@devinplombier  - Perhaps some of the most elaborate internal bracing systems ever construed was the B&W Matrix.  Definitely too much for me but also served as I think inspiration for some really elaborate DIY cabinet construction since then: