I think I read somewhere that piano is one of the most difficulty instruments to reproduce, realistically, from a recording.
For that reason, I have used piano, as well as unamplified acoustic music to 'test' any new audio gear.
I think Mike thought that Bluegrass was a good test of Audio gear.
Fortunately, I am very happy with the way my system reproduces piano, acoustic guitar, human voice.... as well as music with massive transients, like Rock music.
I don't think it's good enough to allow me to 'enjoy' Marilyn Manson though
That would require alchemy. not just audio engineering.