Well my friend you kinda left the floodgates open there when you said, "I would also appreciate comments on ANY planar / ribbon / e-stat design that you think offers a relatively wide sweet spot." You probably won't need your crystal ball to figure out what speaker I'll be pontificating about!
And pontificate at length I shall, spurred on by your request that "If you have ideas as to why this design achieves what it does, i'd love to hear those thoughts too." Make yourself comfortable - we're gonna be here a while.
First of all, let me describe the physical configuration of the Sound Labs. The big Sound Labs, such as the A-1 or M-1, have seven vertical facets, each close to 5" wide, angled at about 11 or 12 degrees relative to one another, so that the seven facets form a faceted curve over approximately 90 degrees. The width of the facets and the amount by which they are angled are carefully calculated to prevent a "picket fence" effect as you move side-to-side in front of them. You see, even at very high frequencies, the facets aren't quite lasers - they do have a few degrees of horizontal dispersion. The net effect of all these facets is uniform radiation over a 90 degree arc, front and back. The faceted curve approximates a line source roughly three feet behind the panels, in some ways remniscent of how the Quad 63/988/989 approximates a point source roughly three feet behind the plane of the panel (though no delay lines are used in the Sound Labs). Neither I nor any of my customers have ever heard any "picket fencing" from the Sound Labs as we close our eyes and move side to side.
Now, I'm going to talk about two kinds of "sweet spots". The first is the imaging (or soundstaging) sweet spot, and the second is the timbral sweet spot.
The width of the imaging sweet spot is influenced by the relative loudness of each loudspeaker (especially in the high frequencies) as we move off the centerline, and the arrival time of the signal from each speaker.
What happens typically as we move to the side of the centerline is this: The sound from the nearer speaker reaches us first, is louder, and has a greater high frequency content because as we move to the side we're typically moving more on-axis of the near speaker, and correspondingly more off-axis of the farther speaker. All three of these factors serve to shift the image dramatically towards the closer speaker.
Sound Labs can't do anything about the relative arrival times as you move to the side, but their line-source propagation characteristic means that the loudness difference between the two signals will be minimized, and their uniform high frequency dispersion pattern (no on-axis "hot spot") means the more imaging-critical high frequency balance won't be tipped in favor of the closer speaker. The center image will still shift as you move off-center, but not quite as much. In addition, sound images in other areas of the soundstage will also tend to shift less.
In my room, I have a chair roughly 12 feet back that is placed up against the left wall, to the left side of the left speaker. From this chair, you can still get a soundstage. The center vocalist is pulled over to the left, but if she's recorded far back in the soundstage, she isn't pulled over as much as if she's miked real far forward (you still get very good depth even from off to the side, because the speakers maintain their superb single-driver phase coherence throughout the listening area, and because the backwave energy promotes the sense of depth and ambience). Instruments to the extreme right still come from over there.
Now, note that the wide radiation pattern of the Sound Labs means that you'll have to take the first sidewall reflections into account - either diffusing or absorbing them, or using a set-up geometry that effectively minimizes them.
Most speakers' tonal balance changes significantly as you move to the side, or up and down. This is because most spaekers' radiation patterns vary significantly with frequency. Not so with the Sound Labs. Their uniform 90-degree front & back radiation pattern (from the bass all the way up) means that the tonal balance is essentially independent of where the listener is. The only exception to this is if the listener is taller than the panels, in which case he'll lose the highs when he stands up because the Sound Labs beam severely in the vertical plane, with the beam-height being the height of the panel.
The correct timbral balance of the Sound Labs even extends into the next room. I always take customers into the next room, leaving the door open, to show them how the speakers generate a timbrally correct reverberant field (which is all you can hear from the next room). In other words, the "timbral sweet spot" holds up pretty much wherever you are, as long as you aren't taller than the top of the panels.
One of the things I really enjoy about the Sound Labs is that if the music so moves me, I can jump up and start dancing around. The soundstaging holds up pretty well, and the timbre holds up beautifully.
For background music, the Sound Labs are really nice (though admittedly somewhat expensive for that role). You see, because they act as a line source, the sound pressure level varies very little with distance. In my room I measured only a 4 dB volume change between 1 meter and 8 meters back (normal point-source speakers have an 11 dB falloff over the same distance). This much more gradual falloff with distance, coupled with the uniformly good timbre, means that you can listen to the Sound Labs at backgound levels throughout a large room - without it being too loud for the people close to the speakers, or too soft for the people far away. This makes it much easier to carry on a conversation while the speakers are playing, if you so choose. And because Sound Labs have world-class clarity even at low volumes, you really can enjoy the music even at only 50 or 60 dB.
The uniform timbre throughout the room, good soundstaging over a wide area, and fairly uniform loudness levels all make the Sound Labs especially nice for group listening and/or quality-over-quantity, non-deafening home theater. The excellent clarity makes voices easy to understand without resorting to a forward, emphasized midrange.
Now, the most holographic-imaging electrostat I've ever heard, and one of the three most holographic loudspeakers I've ever heard, is the metal frame Ultimate 1. That ultra-rigid metal frame makes a significant difference in the imaging, as well as in the impact of the low bass.
If you have any questions, fire away.