Great review as usual.
Very interesting design decision to use that passive radiator. I remember sending him nearfield measurements of mine last year which might have piqued his interest. I'd be curious to hear more about why he ended up choosing it over a vented design. My guess is it allowed for a smaller cabinet which matched well with the active woofer, or even to allow closer placement to the front wall as he alluded to with his BSC. Basically, the same reasons it was chosen by Buchardt Audio. Rick may have other reasons.
Indeed, however another thing a waveguide would do is to broaden the high-frequency dispersion. Notice that the ribbon starts to beam above 10 kHz whereas a waveguide would maintain a more constant directivity throughout the whole tweeter range to 20 kHz. Tradeoffs. Example with waveguide (Buchardt S400):
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I had similar thoughts since I use XT32 to flatten and linearize the bass region but maybe it doesn't matter to us as much. Perhaps making the speaker work well for those not using room correction is the right approach.
Just a guess – likely because you'd have to use 2 of them to get enough area which would be challenging to fit and not increase the size of the speaker.