I dont think that nobody claim that ONLY one instrument will do the job for judging the S.Q. of an audio system... The OP thread speak about a question : is the piano may HELP? indeed it may help a lot for all the reasons many here spoke about...
Some and me too, only said that piano is particularly interesting to use for that...Not ALONE...Voices or chor or anything else will reveal some details or aspect of sound more clearly...No one argue about that.... Then your qualification "altar" of the piano is preposterous and reveal only your own frustration not knowledge or any factual existence of piano idolatry here......
So no, I do not pray to the "piano" altar as some do as the definitive test for accuracy in sound repro.
Second, you description about evaluating speakers FORGET that no evaluation of speakers is DEFINITIVE till the speakers/ room is acoustically under control in a treated room.. For sure for a first evaluation in some seller room a piano cd can help or a chor or a voice cd...It is relative to the listenings experience history of each of us.....
No speakers sound the same in different room especially in different untreated rooms ... Then the choice of piano, voices, etc is mandatory not only to test speakers in a nude room but also to TUNE the speakers/room relation...
Then a decay is not the same decay with the same speaker in different room...A transient and an attack note either... Then when you claim that you have heard wonderful piano on a speaker and shitty voice on the same speaker, this was more an illusion created by the room/speaker specific relation there, because any RELATIVELY good speaker in a well controlled and treated room CANNOT give a wonderful piano sound and a shitty voice in the same room acoustic condition ...
Why ? Because only on a very well treated room and in a well acoustically controlled one a transient is a transient, an attack is an attack and a decay is a decay... WHY ? because acoustician use controlled room to set their standards...They dont use speakers only in any bad room....
An attack note is an attack note. A transient is a transient and a decay is a decay. I’ve heard wonderful piano and shitty voice and have heard wonderful trumpet and shitty piano all on various perspective speakers.
When somebody turn acerbic by lack of attention , i am out....
I wish you could have fun though even with so great need for attention..
😊😊😊😊
When a person garnering attention here turns petulant, I’m out.
Have fun child.