... But since I have your attention, the grill design has proven as problematic as Middle East politics. A one-piece grill is prone to shipping damage and will cover a lot of the veneer. So far I haven't been able to come up with a 2-piece design that looks right. The first draft had an oval upper grill that along with the woofer grill looked like an exclamation point. So I got rid of the oval. It's still kind of klunky. Any suggestions are welcome. As for the "oddly spaced drivers," that's what you get when you optimize the woofer and port locations to maximize the quarter wave contribution, and squeeze the MTM configuration as close together as possible. It's called form following function, or more succinctly, science.
I've never been able to decide which I dislike the most: speaker grills, or speakers without grills. You're damned if you do and damned if you don't. They're flimsy and ugly. One of the things I always liked about the large Advent, especially the later ones with the bullnose rounding on all the edges, is the way that the grill is flush with the front surface of the speaker. From a purely aesthetic standpoint I like the way that Kloss did that, using a big piece of hardboard with big holes cut out of it, and covered in a fabric that was neither too course nor too fine. But to do this, he had to recess the woofer, and the tweeter is left without much real protection.
I do a lot more thinking about building speakers than actually building them, and while I've thought of solutions for most of the things that bug me, I've never come with a satisfactory solution for grill design. If the front of the speaker is made of attractive wood, I don't like covering it up. But if you put separate grills over the individual drivers, it ends up looking like the interior of a car, which is so hideous that only Yamaha would actually do it.
One thing I especially like about the BMR, from an aesthetic perspective, is the way the two midranges and the tweeter are flush with the front face of the speaker. To my eye, this is really, really clean, to the point that I don't have any desire to want to cover them up. But if you were to put a grill on the woofer and leave the MTM uncovered, this wouldn't look right.
As for the functional benefits of MTM, this is an area where again there are differences of opinion. While I'm not personally a big fan, I recognize that there are specific benefits that are desirable so long as the downside is avoided. To avoid the downside, the two midrange drivers need to be close enough together such that their vertical separation is not too great in comparison to the shortest wavelength (highest frequency) within their operating range. You alluded to this need in your comments, and it is apparent upon glancing at the speaker that you perfectly understood this and met the concern by using unusually small midrange drivers and placing them as close to the tweeter as possible. Just from looking at it, I expect that you succeeded in avoiding the downside, and I'll note that it isn't possible to do this if the tweeter is placed within a big waveguide. And the advantages are real. The vertical dispersion pattern is symmetric about the horizontal plane. I don't know what sort of crossover you have used, but even when a crossover design endeavors for the radiation from the midrange and the tweeter to be fully coherent, this can never be accomplished to perfection, and the general consequence will be some amount of upward or downward tilt in the main lobe. Moreover, I believe there is an oft-ignored penalty with using a phase-coherent crossover: the tweeter and midrange each have to be at -6 dB at the point where they are equal in SPL, which implies a -3 dB dip in the aggregate power response. This small dip in the power response has small yet real consequences for the frequency response at all listening locations where coherency does not apply, i.e., at locations where the tweeter and midrange are not both the same distance from the listener. The only way to avoid this, so far as I've been able to determine, is to use crossover slopes where the tweeter and midrange are both at -3 dB at the point where they are equal in SPL, which (I think) implies a 90-degree difference in mutual phase, and which would imply a vertically tilted main lobe, avoided by way of the MTM arrangement.
The sense I get when I look at this speaker is that you had an intimate understanding of several design issues that are ignored in most speakers produced by major manufacturers, that you invested a great deal of time and effort in coming up with solutions that work.
Except for the damned grill.