Interesting thread! For my part, I have been quite impressed with a number of modern horn systems, and especially enjoyed attending the Midwest Audio Fest this year.
My (ever-changing) theory of loudspeaker quality is this: A loudspeaker must do something magical (timbre, soundstaging, dynamic impact, etc); and then it must avoid distracting you from that magic. A speaker that does nothing wrong but nothing magical won't draw you into the music, and a speaker that has (for instance) magical imaging but a boxy sound will destroy the illusion almost as fast as it creates it. While we all pretty much agree on the magical aspects, our sensitivity to different sonic aberrations varies significantly from one person to the next - therefore, we have widely varying opinions about which speakers sound "best".
A high quality horn system can indeed be magical. While I do not presently sell a line of horn-loaded loudspeakers, I hope to change that before too long.
Horn speakers are usually hybrids - dynamic woofer with horn mid/tweet (exceptions include corner horns like the Klipschorn and Hartsfield, Burce Edgar's Titan, and Agantgardes with the opional bass horns). The discrepancy between the radiation characteristics of a dynamic woofer and a horn presents a challenge. JBL did a superb job of integrating a horn with a woofer in the old 4430 (and subsequent derivatives) - they matched up their respective radiation patterns at the crossover point. Among current generation hybrid horn systems, the SP Technologies line is to the best of my knowledge unique in fully embracing this approach (though they trade off high efficiency in exchange for deep bass in small boxes). Classic Audio Reproductions speakers also do quite well in this area.
In my experience, Avantgardes are extremely lively and dynamic, with superb imaging and pretty much negligible horn coloration, but I can hear the transition between woofer and horns. The wide pattern of the woofers puts lots of low-frequency energy out into the reverberant field, while the narrow pattern of the horns puts relatively little mid and high energy out into the reverberant field. This can make the bottom three octaves or so sound a bit slower and thicker than the rest of the range.
Like JohnK, I have heard very precise imaging from the Avantardes. I personally prefer the considerably wider sweet spot and more enveloping sense of ambience you get from a well set-up pair of Sound Labs, but that's a matter of personal preference - obviously John perfers the Avantgardes. Not all horn systems image well, though, and in my experience they tend to have a fairly small sweet spot both in terms of imaging and correct tonal balance.
On the other hand, a good horn system typically gives you superb dynamic impact and liveliness and sense of presence. This is magical. And as long as you aren't distracted by their relatively minor little aberrations (no system is perfect), a good horn system can certainly give you goosebumps. I tend to prefer a good planar or a good horn system to a good conventional direct-radiator dynamic system.
I would like to offer my customers very fine loudspeakers at seemingly opposite ends of the spectrum: full-range planars and high-efficiency horns. I haven't settled on that horn system yet, but I'm still lookin' and listenin'.
Duke
