Been doing this for 3 decades professionally. I have heard some very flawed loudspeakers sound pretty darned good on piano. If I had to reliably test a speaker in a short period of time piano would not be one of the first 3 albums I would put on. If piano is really important to you then certainly run it by any speaker you are thinking of. But use other music to flesh out flaws that can be hard to hear on piano recordings.
I have had a speaker perform beautifully on piano and organ and fall apart on upright bass. I've had good piano repro with horrible, shrieky violins. Good piano and clumsy mid to tweeter transition. Good piano and ear sawing trumpet. Good piano and cloudy voice.
I have had a speaker perform beautifully on piano and organ and fall apart on upright bass. I've had good piano repro with horrible, shrieky violins. Good piano and clumsy mid to tweeter transition. Good piano and ear sawing trumpet. Good piano and cloudy voice.