A casual look into a piano will reveal many stings that get hit by felt hammers to produce sound. One might think that the sound source is the strings. Actually, behind the strings, and below them in a grand piano is a sound board. This is a large area of thin wood, and its name indicates its purpose. The tonal quality of the instrument depends very much on the quality and condition of this wood. Note that the sound board resembles a planar loudspeaker.
I have noted that loudspeaker fidelity is often improved when the loudspeaker resembles the instrument in some way. This is most obvious with trumpet reproduced by a horn driver. I have also noted that the titanium tweeter in a small B&W speaker that I have makes violins (with metal A and E strings) sound very real. So, according to my theory, planar speakers have a head start in reproducing piano.
