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Unfortunately, it doesn’t seem much like the digital piano makers take into account any of what you’ve said with speaker placement or choice.
And while good monitors cost that much for the consumer, I think digital piano/keyboard makers would probably get a much lower price in production when they just have to buy the internal parts, and would be buying at scale. Most of the major companies pay pennies for what the consumer spends dollars on.
As for the stereo image, I’m less referring to a sense individual notes coming from a particular direction, and more referring more to the entire wall of sound that comes from the instrument. The sound from a piano comes, as you pointed out, from the very, very large soundboard, and also from additional resonances in the actual body of the instrument. When you reduce that to a few tiny speakers, that overall sense of sound is loss.
Headphones help with this, as do properly calibrated and aimed monitors. Speakers, also properly calibrated and aimed inside the body of the instrument, especially those which benefit from aiming, as well as overall proper placement would help with creating that sense of sound.
As it stands, the vast majority of digitals do not do well at recreating that sense of sound.
Of course, no digital is going to sound like a real piano at a 1:1 level, but I do think more speakers and better speaker placement would do a great deal toward closing the gap more.
And, of course, one wouldn’t expect this in every digital - the entry-level and medium-tier ones probably wouldn’t have this. But for the ones costing into the several thousand, especially those aiming at great authenticity, I think this should be par the course.