How does solo piano help you evaluate audio gear?

Well-recorded piano can show a lot about a system. Assuming it was well-recorded. It covers virtually the entire musical pitch spectrum and has a huge amount of weight when captured accurately, not to mention the inner detail of the hammers striking the strings, the sounds of the pedals and the bench creaking, which it often does. So, assuming you know what to listen for, piano can be an excellent tool in selecting a speaker. But it is one of many tools. If I were forced to pick only one, I personally would choose the human voice.

Power, weight, and scale.

It's the most difficult instrument to capture.

Good points and very helpful. I went out of my way *not* to ask if it should be the only one. I'm really drilling here for phenomenological details, i.e., descriptions of what one notices in a piano recording that has probative value for a more general conclusion about the capabilities of the system. There is some of that in this post by McGowan.

Acoustic instruments go a long way in the realm of realism when a system is firing on all cylinders. If the initial movement (of a key, valve, etc.) is captured, followed by the strike of it, followed by the burst and then bloom, and then the associated spread, decay and reverb, the whole soundstage develops to a degree that can make one momentarily forget they're listening to a recording. A piano is a great instrument that can do that.

One can faithfully replicate the passing of a heavily laden semi without all the shaking when a piano plumbs its depths. A microphone placed close to the soundboard can be all it takes to make you forget and just revel in it all.

If your system can already capture and recreate that kind of moment, then you're almost there.

All the best,
Nonoise