EXAMPLES
With the revival of the old tunings musicians began to realize that the Chopin that they’d grown up playing was not the Chopin that Chopin himself had composed.

No. 1
Because if you play the "Heroic" A-flat polonaise on a piano tuned NOT "perfectly" but imperfectly the way Chopin’s piano was tuned, it is a different, indeed, truly heroic, shockingly brash and bold, piece of music. The move, for example, from A-flat to C-sharp in the middle of the piece and then back to A-flat at the end makes sudden emotional sense instead of being mere aural titillation: It is tripartite aural magic—the hero announces himself in A-flat, pauses to gird his loins in C-sharp, then goes forth to do in A-flat whatever heroes do.

If this seems complete nonsense, that only indicates how thoroughly conditioned your ears are to equal temperament.

No. 2
Another, simpler example: Chopin wrote 28 mostly very short "preludes" in the various keys. Some are fiendishly complex and come and go almost before you realize that something extraordinary has happened. Some are deceptively simple.

The e-minor prelude (op. 28, no. 4) starts off slowly, longingly, and properly in e-minor. Then in its brief two-minute span Chopin starts wandering away from that key. It’s as if there’s some distant magnet or powerful source of gravity that’s pulling him off, away from, out of his deep nostalgia. Confused, searching, he goes in and out of various modulations, to no avail. The deep pull of e-minor is too strong. Then abruptly, four measures from the end, he finds his way for just a moment to the source, to purity, and we suddenly get a C-major chord. On a piano tuned in other than equal temperament, the unexpected purity and simplicity of this chord is shocking. In this thicket of confused e-minor remorse, the C-chord is like glimpsing the godhead. But Chopin can’t hold it, and the piece quickly sinks back toward e-minor and fades away nostalgically toward silence.

All of that, my friends, is missing, unless you have a very good aural imagination, from any performance of the piece on an equal temperament piano.

And the same applies to the entire much-revered canon of so-called classical music.