@pinwa
Panel speakers also have relatively narrow sweet spots
Well, that may be broadly true, but certainly does not apply to Quad's electrostatic panel speakers from the Quad ESL-63 and all subsequent models.
The reason is simple, but the execution is not! Note that the ESL-63 was conceived in 1963 but took 18 years of development before it was released to the market. So this type of panel speaker has been around for over 40 years but the way it works is still poorly understood.
The design principle is to emulate a single point source of sound, radiating spherical soundwaves from an imaginary point about a foot behind the centre of each panel.
Each imaginary wavefront reaches the centre of the panel first, as it travels at the speed of sound from the imaginary point source. Then it appears to expand on the panel in a growing ring.
The Quad design emulates this behavior by organising the stators as eight annular, concentric rings. The signal is progressively delayed from the innermost to the outermost ring. The delay required depends only on the geometry and the speed of sound. The net effect is all the benefits of an electrostatic panel plus the coherent benefits of a point source, giving amongst other things a huge sweet spot.
Quad did this with inductive analogue delay lines, but these days a digital solution would surely be easier. You would need 8 DACs and 8 amplifier channels per speaker!