Suffering is relative to your expectation and perception. Still, take the best driver you can imagine, measure the distortion, and you see same pattern but better than any standard compression driver: all distortions are phenomal low, except of the K2 standing out. Which is still by far better than any standard speaker yes, still its the only one standing out. Kiling this, you may achieve a distortion on the speaker as low as an amp! Even lower than most SETs.
Another point, good drivers have a specifically designed rearchamber. One of the most amazing drivers I know is the Fostex FD200 which uses a dome rearchamber in exactly the same shape of the diaphragm, just a tenth of an inch away, creating a tini tiny volume to create a complementary pressure to the horn load. Klipsch did the same on the Korner. So there is some relevance of the rearchamber with proven influence on sensitivity around the cutoff frequency, impedance and even distortion!
Also to say reflections and other things might be worse are a reason not to improve it, doesn't make much sense yet. I also got back ache some days, still im interested in improving this.
Only thing I have to take serious is the volume of air between the drivers. You are right with that hint. On the subwoofers the compound is proven to lower the fs by the aircoupling, due to the MMS influence. Still, as we use the double the engine (coil+magnet) behind, its balancing it, as mass can not be viewed separately from the engine. We know this by cars. Look also at electrostatics, even 10 squarefeet drivers will reach 40.000Hz with ease, as the engine is as big as the membrane, so size doesn't matter.
Take a dodge challenger of 3 tons weight but a V8 with 450hp, its got no chance against a british concept of 2 liter engines with half the hp, but third of the weight
I rather see the influence of the fs in the subwoofers due to the reduced VAS, which enlarges the case size in relation to the equivalent of the driver. That lowers the fs. It also means less efficiency. Which logically comes from running 2 drivers, but only 1 output. So as I said, the amps gotta work double. Only downside I can see at this point.
Unless youd find another flaw I didn't see here yet. Appreciate your critical view anyhow!