Tweekerman, you are right in that there is a shortage of excellent quality dedicated midranges. Most high-end speakers these days use a woofer, a midwoofer, and a dome tweeter. One problem that these systems tend to have is a narrowing of the radiation pattern in the upper range of the midwoofer. But with the 3" diameter ATC dome crossing over to a dome or possibly ribbon tweeter, it is possible to maintain a more uniform radiation pattern through the crossover region.
Why in the world would this matter? Because a characteristic which generally distinguishes live music from reproduced music is how natural the reflected, or reverberant, energy sounds. What we call "off-axis" response determines the tonal balance of the reverberant field. And the tonal balance of the reverberant field is a significant factor in both timbre and listening fatigue. Basically, you want a fairly well energized, diffuse reverberant field that has the same tonal balance as the first-arrival (on-axis) sound.
And the ATC dome midrange has more uniform reverberant field response than a larger-diameter cone would, without having the dynamic non-linearities that plague other dome midranges which don't have that double suspension system.
The emphasis on flat on-axis response stems from research done back in the 60's and 70's. It was determined that the on-axis response played a greater role than the "power response" (summed omnidirectional response) in establishing the perceived timbre of a speaker system, and flat on-axis response has been the holy grail ever since. But another way to interpret the data is temporally: the first-arrival sound plays the dominant role in determining timbre, but the reverberant sound still plays a significant role. I believe that minimizing the discrepancy between the first-arrival and reverberant sound is a worthwhile pursuit, as this mimics the characteristics of live music.
Anyway, I'm not saying the ATC is necessarily the best midrange by all possible yardsticks, but I think it's probably among the better units of its type. I've heard the smaller ATC speakers and think well of them, though I did not choose to become a dealer (I could still hear a bit of boxiness, and for that much money I prefer speakers that don't let you know the music is emanating from boxes). I've actually never heard the ATC dome midrange, so my comments here are educated guesses at best and SWAG speculation at worst. As I write this I have a pair of ATC dome midranges on order to dink around with - perhaps when I've had a chance to get to know them well, I'll have something more useful to post.
Best of luck to you in your brave and bold projects, intrepid Tweekerman!
