How does solo piano help you evaluate audio gear?

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A pianist friend just recommended this article and pianist to me, knowing that I'm presently doing a speaker shoot-out. My question to you all is this:

How important is solo piano recordings to your evaluation of audio equipment -- in relation to, say, orchestra, bass, voice, etc.? What, specifically, does piano reveal exceptionally well, to your ears?

Here's the article:

https://positive-feedback.com/reviews/music-reviews/magic-of-josep-colom/