Learsfool is right on the money. And his opening remark is particularly
useful; even if he made it partly in jest.
I would add that the piano is best from the standpoint of using an
instrument with the most extended frequency range. That is obviously an
important consideration when auditioning gear. Also, if the gear is a
turntable, nothing like checking speed stability with piano decays. Now, as
far as timbre goes, I have always had trouble with the notion that the sound
of any one instrument is more difficult to reproduce than another. And that
takes me back to Learsfool's opening remark.
ALL instruments (and voice) are difficult to reproduce accurately; and I
would suggest that they are all equally useful in assessing the ability of
gear to reproduce timbre. It all depends on which instrument (or voice) we
are each most familiar with or have the most affinity for (maybe the same
thing). True, some instruments have a more complex tone than others. But,
does that not also mean that the ones with the less complex tones will be
more difficult to differentiate? IOW, as Learsfool states, the horn (his
instrument) has one of the most pure tone structures. I am sure he would
agree that, as a result, the differences in tone between each member of his
horn section are that more difficult to differentiate than, say, the oboists'
tones. But the differences are certainly there; and to Learsfool, very
obvious; because he is intimately familiar with the sound of the horn. So, it
follows, that gear that can reproduce the subtle differences between
various horn sounds will, as he points out, easily reproduce obvious
differences.
BTW, while I agree that the reproduction of timbre is key for realism, I think
that most gear has gotten good enough that an even more important,
elusive, and unfortunately overlooked aspect of music playback is dynamic
nuance. As far as I am concerned, that is the area that separates the men
from the boys (in gear).