How does solo piano help you evaluate audio gear?

Not to be facetious - I think solo piano is an important instrument to evaluate equipment with if you like to listen to a lot of solo piano.  I would venture a guess that the majority of audiophiles cannot, either by budget or space constraints, own no-compromise speakers that excel on all genres of music.  Assuming such a thing exists.  So most of us make some compromises and IMO, that's best done by choosing speakers that make the music you value sound good to your ears.  I'm not saying "accurate" or "true to the recording" because, unless we produced the recording, we can never know what that is.  So I'm just saying it has to sound good to us. So for me, piano is important.  BTW - If you want some nice piano music, try Bach on a Steinway and Dr. John Plays Mac Rebennack.  Both excellent solo piano recordings, IMO> 

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.