If i could pick only one instrument to test timbre rendition i would pick a piano...
Because the sound of a live piano is well known at least as guitars, and the way the transients attack goes on a piano is very dynamic and the tone timbre decay so complex , the bass so deep ...
Piano sound is ideal to test acoustic of room or playback system embeddings ... It is very hard to set a piano right for recording as well as for playback in a room ...
Then to test imaging and soundstage , i use an audiophile level recorded opera : Kurt weil with Lotte Lenya " the three pennies opera" ...
The recorded acoustics is stunning, with the singers walking and turning their heads, and in a good listener acoustic room , as my last one, they moved all around me and beside me , as if they were NO SPEAKERS ...
In my new room i am nearfield with smaller speakers , the soundfield is stunning but less encompassing of my listener position , because so well is my acoustic corner well done , it cannot compared with a room including 100 Helmholtz finely tuned resonators ..
This is the best interpretation and the best recording ... Astounding ...
https://www.youtube.com/watch?v=qR33bL5aNTk&t=2244s