First--before I start disagreeing and sounding like a Grinch--thanks very much for the thorough review, and the polished prose. You pretty much described the speaker I intended to design (with Paul Kittinger's help), which is very gratifying. But I'm not going to let the 2-inch-thick acoustic guitar analogy go unanswered, or the corresponding theory that the ST would somehow sound more realistic if the cabinet walls were thinner. I'm only taking you to task because this goes to the whole (and critical) distinction between music reproducers (speakers) and music producers (instruments). String instruments sound the way they do because the wood panels, and subsections, are vibrating in particular modes, producing a unique pattern of overtones. If you play a guitar or cello back through a speaker that is adding its own vibrations to the sound, it won't sound more realistic. It might sound "warmer," or less distinct, but not more realistic. Not "more in the room."
At the same time, I'm not taking issue with your observation that the ST's fall down a bit in the "disappearing act" test. This may be in part due to the crossover topology, which is a very straight-forward 4th order Linkwitz-Riley acoustic slope. Although I think the crossover is carefully optimized to place the tweeter and woofer in phase at the crossover frequency, they are a full cycle apart. That's just a feature of this crossover type. Further, this is an MTM design, with it's unique radiation patter. It can produce a very focused sound, but not in my experience one that seems to be "floating in air." There aren't a lot of crossover choices in an MTM implementation. If you try and achieve true 1st order slopes and time-align the drivers, there will be massive interference effects even slightly off axis. Finally, the ST doesn't harness the midrange rear wave using a rear opening to increase ambiance. It can't, because that would ruin the bass response.
I suspect you would find Jeff Bagy's open-back speaker (can't seem to recall the name of that thing) much superior in the "floating-in-air" department. And Jim and I are working very hard to come up with a truly full range speaker (down to 20 Hz or below) that will also pull off this trick. It won't ge a 2-way, and it won't be cheap. But it will have really solid cabinet panels. Anyhow, thanks again for the great review.