Let's first acknowledge that it's hard to generalize about the sound of speaker types. There is a wide variety of horn speakers just like there is a wide variety of cone speakers and electrostats. That out of the way, the single most compelling quality of horns, done right, is dynamics. As a class, horn speakers have more sudden starts and stops to music than do cones or electrostats. This natural ebb and flow of dynamics can allow horns to sound more real and more lifelike than other types.
The difficulty is achieving this unique dynamic ability without giving up some other qualities that cone speakers seem to achieve without difficulty. For example, some people, myself included, are very sensitive to horn colorations like a "shout" or glare. Nearly all direct radiation horns seem to suffer from this to one degree or another. In addition, it can be difficult to mate a horn to other types of drivers. Achieving a smooth, coherent blend of drivers is more difficult when one of the drivers is a horn.
I am fortunate to have a set of speakers that thread the needle between capturing horn dynamics without sounding like a horn. My speakers are a 2-way inspired by the Western Electric 753 monitor speakers. They use a horn from 1200 Hz on up and a 15" dynamic woofer in a bass reflex cabinet. The horn is fairly compact but it is unusual by not having direct radiation. The horn is actually bent into a 90 degree right angle; the horn exits on the front panel just above the woofer but the compression driver inside is pointing upwards. I don't know how much the shape of the horn accounts for the resulting sound quality, but I do know that this arrangement does not have the dreaded horn "shout" or any other colorations that to my ears identify it as a horn. In fact, the horn blends so well with the 15" paper cone woofer that it sounds more coherent than any other all-dynamic multi-way speaker I have used. The tradeoff is that my speaker does not have quite the most dynamics or the largest soundstage of some other horns. For example, I heard a 3-way consisting of a Western Electric 22 horn with 555 driver, a 15" woofer similar to mine in a large sealed cabinet, and a Jensen 302 horn tweeter on top. The dynamics on this speaker were noticeably more dramatic than on mine, and it was also more detailed and more spacious. All very good things, but....the WE setup was not as coherent as mine in any of the 3 aspects of coherency that are important to me--- tonal balance, dynamics and soundstaging. The 3 drivers in the WE speaker all sounded different from one another, and the resulting sound while uniquely impressive in many respects was actually less satisfying overall than my own speakers. So it all comes down to finding the right compromise for a given listener.
