It's interesting that the examples given so far have been the human
voice - obviously critical for timbre. funnily, the instruments I'm
most sensitive to are the woodwind - clarinet, oboe, bassoon, flute -
the piano and in particular the jazz drumkit. Since I was a jazz
bassist for many years I used to stand right next to the kit for hours
and hours on end, and the sounds of hi-hat, snare, ride and crash
cymbal, toms and bass drum are etched on my brain. When the drumkit is
right, top to bottom, there's not much wrong with the rest of it.
Cymbals in particular are a real test of an audio system. So are the
toms - they have a particular reverberation which is very distinctive.
Andy