These observations seem consistent with what Franz Mohr said in his autobiography (at least in the excerpts I've read). If I remember correctly, he strongly disputed claims that Horowitz ever requested, or got, any special modifications to his piano. In fact, Mohr gave the impression that Horowitz wasn't particularly demanding about the instrument -- though he seems to have let his stage fright spill out onto the technician from time to time. This contradicts a Horowitz biography of some years ago which claimed the treble hammers were made hard as glass at the maestro's insistence. Given that that biography also made a lot of baseless allegations against Horowitz (for example, that he had a very limited repertoire), I'm inclined to believe Mr. Mohr. Mickey Kessler
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