Thoughts on the most difficult instruments for speakers to reproduce?

String sections, and yes violin higher registers.  It seems almost impossible to find sound reproduction that captures the combination of texture, bite and silkiness of real strings.

And most speakers tend to thin out the body of the sound as you move into the higher frequencies.  Drum cymbals in real life are big and round, never the tiny pin-points of bright sound as through most speakers.

Piano...yeah...always on the list.

Let's hear why piano is so specifically hard to reproduce.

I'd say some reasons:

1. Spans more of the frequency range than most instruments, hence has wider scope to show weaknesses in upper and lower areas of the speaker design.

2. Has a combination of percussive yet soft quality (a soft "hammer" striking hard wires) that is so easy to miss, either becoming too fuzzy or soft, or too hard and artificial.

3.  It seems extremely hard to both record and reproduce the BODY of the piano sound.  In real life the whole instruments seems to be in play, and you can "feel" the weight of the vibrating strings and the soundboard, body of the piano.  On recordings and through hi fi systems piano becomes a set of detached, floating keys being struck...as if severed from and preserved, the rest of the instrument thrown away.