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

@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:

@erik_squires --

Exciting project. 

Lets stop talking about subwoofers and talk about this project as a new 3-way speaker system.  Instead of thinking of adding a subwoofer and crossing around 40-80Hz I’m now thinking 150 - 300 Hz. [...]

As a fully active approach and using the same digital XO/DSP over the entire frequency span you’re dealing with, I certainly consider such a system to be a single x-way speaker system per channel - even when using separate subs (via the same active XO) crossed below, say, 100Hz. The main point here is that each speaker channel is treated as such through the active crossover (with on-the-fly settings adjustments via a control unit/PC/laptop from the listening position), and that you’re using a dedicated approach to both high- and low-passing (etc.) from the low frequencies on up, and not merely "latch on" active subs to an existing and not high-passed passive main speaker system - the by far most common approach. 

Generally speaking I want to cross my new woofers as high as possible while still maintaining excellent off-axis response, not something a lot of commercial speakers have.  In other words, I cut them off before they start to beam.  Having the woofers located under the mid-woofers means I can cross higher without fear of the ear locating them, unlike a subwoofer appliance.

Maintaining a fairly flat power response is an important trait, not least in the main speaker driver transitions. Using waveguides or horns this can be achieved especially well crossing over to the woofer/mids, and at a lower XO frequency compared to using direct radiating dome tweeters. I typically don’t find the latter to blend well with woofer/mids below them, not least when compared to using waveguides or horns.

In the lower frequencies I always recommend placing a pair of subs symmetrically and close to the mains and coupled in stereo as a general rule, with the best case scenario being a fully active approach with high-passed mains and all. A mono’ed DBA has something else to offer, but while having an acoustically less smooth response (unless corrected with DRC) I usually prefer a dual sub setup as described. 

By crossing high (150-300Hz) I reduce the load on the mid-woofers, increasing dynamic range but also has the extremely beneficial side-effect of reducing audible distortion even while playing low.  

Indeed, but this is also dependent on the type of mid-woofers used. Using high efficiency and larger diameter pro segment woofer/mids (12" on up), not least in pairs per channel, they’re turned into rockets with plenty of headroom in a domestic environment when high-passed no higher than 80-100Hz. Crossing above ~150Hz is entering the power region (up to about ~450Hz), and this is also a sensitive area with regard to placing a crossover point, I find, and how affects the lower midrange and overall uniformity of presentation here. Oh, the compromises - both here and there.

What is special or innovative about your design?
What makes it stand out compared to existing approaches like push-pull, isobaric, dual woofer/passive setups, or other double front-firing designs? What are the strengths and unique aspects of your solution? Even DSP-enabled features already exist—there’s no servo control?

How do you address the time alignment issue in a vertical double front-firing design?
Why not use a horizontal layout instead?

What makes you believe this is a waxing poetic, standout piece worthy of Stereophile’s attention?