A bit more detail:
I tried that first dual woofer sub in three different locations at the front of the room, and saw very little difference in the measured output. But that first measurement and the fullness of the bass produced encouraged me to keep going.

In all instances I had to have the tone control on the Yamaha receiver set to boost the bass. While I could do that in Equalizer APO, that generally means cutting higher frequencies to get enough boost in the bass region. The bass control on the Yamaha seems to be doing a very accurate and natural sounding lift.

Without the bass boost there wouldn't be enough bass output from the 2 woofer setup.
The 4 woofer dual subs that I settled on can be improved by adding a lowpass to 2 of the woofers and using those exclusively in the very low bass range (40Hz and down), while the other two operate up to 80Hz. This should extend the bass output even lower, but does complicate things.

You'll often hear that open baffle speakers don't excite room modes, but I saw no evidence of that. The one distinct advantage I saw was that they are easier to place in the room - boxed subs will perform much differently in different parts of the room, while these work the same (more or less) wherever you put them (within reason - they do need to be away from the walls by a certain amount).

As for the quality of the bass, I couldn't hear any difference other than the struggling problems of the woofers pushing their excursion limits. Keep in mind that this is crossed over at around 80Hz.
My isobaric sub was putting out solid, effortless base that sounded accurate and extended down to 20Hz. These are the same except for the depth of the low end, but sound identical to the isobaric sub. My room is 14' x 14' and is well treated acoustically, and these subs easily provide enough thunderous bass for modern movie soundtracks.

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