@audiokinesis --
This is almost exactly what I’m working on, and had hoped to introduce in 2020 but... stuff happened that year...
Anyway I designed a large-format Oblate Spheroid waveguide using Earl Geddes’ equations, like you targeting a 700 Hz crossover to twin 15" midwoofers. That 700 Hz figure is consistent with the findings of David Griesinger which Geddes subscribes to, and is very close to the 800 Hz crossover that Greg Timbers uses in the JBL M2. Imo the ability to cover the spectrum from there on up with a single driver is a major advantage over more "conventional" approaches, in addition to the other advantages of large drivers and high efficiency.
And of course the way around the bass extension/box size/efficiency tradeoff relationship is to hand off the bottom couple of octaves or so to subwoofers.
I’m rather surprised by how similar our approaches are. I knew we were barking up trees in the same forest, but didn’t realize it was the same tree!
That is certainly interesting re: the similarity of approach, and thanks for your added/confirmative info here! Indeed, covering the whole frequency span from ~700Hz on up from a single driver/waveguide/horn element appears to be paramount. The current driver/horn constellation (and soon to be fitted with a bigger horn) of my main speakers sport a 2" exit with a 3" titanium diaphragm, and thus lends great energy and "breathing room" to its lower to central region. This does affect the upper octave however compared to smaller exits of 1 and 1.4" which don't roll off quite as early, though conversely at the expense of lower band energy and higher distortion here. Choices, choices; it's a matter of balance (and preference) with the implementation at hand, but I find it's worth the effort compared to adding another driver element, cross-over and point source.
If I may inquire: what's the intended waveguide exit size of your upcoming design, and would the twin 15" bass/mids be configured D'Appolito style or with both of them below the OS waveguide? Btw, I'm thinking whether Timbers would've preferred a slightly lower cut-off than 800Hz with the M2's, but that the size of its waveguide simply won't allow it? Scaling up the size here likely would've made for a bulkier, and less commercial appearance.
To the OP: sorry for veering off-topic. If nothing else what's elaborated above is an indication of a preference and a desired high eff. design path that seems less popular or visible not for reasons of lack of sonic prowess, but rather size requirements and design principle in particular. Few audiophiles appear interested in compression drivers and horns/waveguides, not to mention larger pro woofer/mids that extends into the central midrange; I'd wager it's largely conjecture aimed at a speaker segment that doesn't speak the conventional hi-fi narrative, and where auditioned their typically denser and more direct/present sounding nature mayn't appeal to those who're usually exposed to a leaner, more laid-back and softer/reverberative presentation.
And may I just add: high efficiency doesn't automatically equate into easy or easier amp load. Less power is stored into heat for a given SPL, but a complex passive x-over here can still drain amps with less prodigious power supplies. For easier an more optimal amp load active configuration is required.
