Why not horns?

" I have yet seen or heard any speaker which can play loud with such low 0.3% THD. "

"ATC claim this on their highest end designs such as the SCM300 while playing at 121 dB SPL (definitely loud). I am skeptical that they can actually achieve this down to 20 Hz but I imagine it is not far off the truth especially with a 6 db bass boost from a typical soffit mount - certainly from 40 to 20 Khz it can probably be done. Of course, at normal volume levels the distortion is probably even slightly lower but 0.3% THD is definitely approaching the limits of speaker technology even at moderate volume levels."

I find this interesting as the baffle in the "soffit mount" is what gives you the 6db of gain. This is the same property as a horn with all of its issues. Soffit mounting done properly is useful. Improperly done, and it can be just horrid no matter how great a speaker you put in there. Besides working out proper placement, the fabrication of the soffit itself is very important as it becomes a transducer along with the speaker.

Macro, I understand what you are asking but you could open up a can of worms with some of the teckies on the site. When You listen to music essentially you are listening to distortion.
I have a friend that simply talks about specs and #'s. Since the 70's numbers have been meaningless to me. Remember the high powered receivers with .00000001% THD? If you put that receiver next to a Dynaco ST 70 from the 60's you may have a totally different opinion of those numbers. I believed in all that until I heard my first tube amp. Even at 2% THD I HEARD ONE THING, MUSIC!
NO numbers could add up to that no matter how low they were.
The sound regardless of how it is reproduced is key. Whether it be horns, electrostatic, dynamic or ribbon speakers.
I am partial to ribbon technology. I do however like horns. Some setups are a tremendous value and in some regards better others. Once synergy is reached there are many types of reproducers that can please you. The trick is how long will those horns keep you interested or impressed. What are you going to do when you hear another system that is superior to yours using your own ears?

What pleases your ears will surely appeal to others but three will always be skeptics regardless. Just Ignore them. It's all what you think. I have Klipsch on my Hot Rod Computer. Before that I had Monsoons. I love the Monsoons but there is something about the Klipsch where the sound is more dynamic and authoritative. Even though the Monsoons were dynamic too the Klipsch were even more so.
Unfortunately, my main listening room will only accomodate speakers with a limited bass response to around 40Hz. I still have 2 Dali Suite 1.2 Subs that augment the bottom end. They are both Acoustic Suspension and there isn't much bloat. So far it has been pleasing to the ears of all that have listened to it.
I have been trying to find a pair of LaScallas for my 14 x 13 foot room for the past year locally so I can pick them up. Personally everyone says they will be too big for my room. What if I discover that midrange as you have. I'm only going to say what have I been waiting on all these years. All this to say, horns have some very natural properties in it's presentation the midband is what they do very naturally. Some will be in for face and some will hoot at you. You room is very important. Right now, Ribbons are still my favorite designs but until I get some horns I won't know what they can really do. I'm still hoping I can find a pair locally. I use to sell them in the 80's and I grew a liking to them but they were out of my budget at that time and I didn't have the space for them. What you hear is all that matters and don't let anyone tell you that they don't sound good. Forget about the distortion numbers, etc. If you are listening to tubes you are most like above 1% distortion anyway. Listen to the virtures of the music and how it communicates the music to you. The Hype of all this is that we get caught up in numbers and Audiophile terms and we forget about what this hobby is all about. The music.

Why isn't everybody using recent model Quad electrostatics? Single driver, no crossover, low distortion, etc...

Those of you who reject all horns out of hand have much to learn.

i always think of horns as like putting the sound under a magnifying glass. A lot of things do not look good under a magnifying glass. Some things can be beautiful but it it may take a while to find the right sound under high scrutiny.

In other words, it may take a good bit more work and precision to get horn loaded speakers to sound "right" compared to less efficient designs that are more forgiving.