Sean, what an incredible post! You did a superb job of laying out the merits of horns and 'stats without becoming "partisan".
I'd like to suggest that the dipole radiation pattern of a full range electrostat is actually an asset rather than a limitation. The only significant placement issue is getting them far enough out from the wall behind them, and five to seven feet is usually adequate. You might want to treat the first reflection points with a bit of absorption or diffusion, then fine tune the toe-in angle, and you're pretty well done.
A good full-range dipole has three significant advantages over a direct radiator loudspeaker:
1. The bass is significantly less colored by the room.
2. The direct and reverberant fields are more alike.
3. The sound field more closely approximates a live event.
A dipole does not excite room modes more than a monopole speaker - in fact, it excites them far less. This is because a dipole's figure-8 radiation pattern puts 5 dB less bass out into the reverberant field than a monopole's spherical radiation pattern. True a dipole will excite the room's front-to-back standing wave modes (and so will a horn), but it will put much less energy into the side-to-side and vertical standing wave modes than a direct radiator does. The result is much better pitch definition, because the room isn't contributing nearly as much overhang to blur the decay of the bass notes.
Because a well designed dipole (such as the Maggie 3.6, 20.1, and full range Sound Labs) maintains pretty much the same radiation pattern all the way up and down the frequency range, the reverberant field will have the same tonal balance as the direct sound. This is conducive to natural timbre and to long-term listening enjoyment, because the brain expects the reverberant field to sound like the direct sound with the room superimposed on top. A direct radiator almost never gets this right because its radiation pattern changes drastically as we go up and down the frequency range. Therefore, its reverberant field cannot possibly have the same tonal balance as its on-axis sound. The result is listening fatigue over time. You can listen to a good dipole literally all day long and never get tired of it (the freedom from boxy colorations probably helps here as well).
A dipole that approximates a line source sets up a very different kind of sound field than a horn or direct radiator can (with the exception of line source direct radiators, like the Pipe Dreams - which have problems of their own). You see, sound pressure falls off with the square of distance from a point source, but linearly with distance from a line source. Okay, once again, in English: As you move farther back from a tall dipole speaker, the volume falls off much more slowly than when you move back from a horn or direct radiator speaker. The result is a vastly different "feel" than what you get from a conventional speaker. This uniform sound field feels more like a live performance, because as you listen to a live performance from forty feet away, the sound field is very uniform in volume and tonal balance.
Alas, Sean, I don't know as much about horns as you do about 'stats, so I cannot give the comparison as even-handed a treatment as you did. I certainly agree that horns do macrodynamics better, are easier to place, and are much easier to drive. So far I have yet to hear a high end hybrid horn system where the direct radiator woofer matched the clarity of the horn. But, in all fairness, I have yet to hear a high end hybrid electrostat where the direct radiator woofer matched the clarity of the panels. Just as the ultimate in electrostats is the full range panel, I would imagine that the ultimate would be a full range horn system. Besides the K-horn, does anybody make one?
