Thoughts on the most difficult instruments for speakers to reproduce?

A speaker system with less than ideal crossovers will ruin piano every time.  I have hybrid ESLs with the bass woofers crossing over at about 500Hz, pretty much right in the middle of the keyboard.  Yikes!  Thankfully, I simply can't tell when the notes cross over and the timbre never changes.  Well done.  

I heard a respected 3-way system once playing piano and it was super obvious when the notes switched drivers.  That experience has since made piano my "tester" for speaker integrity/reproduction. 

@wspohn -  "I tried it later on my Electrostat system and it works there too - pretty sure it wouldn't if it was a hybrid speaker mating a cone woofer with the main panels."

My JansZen Valentina hybrid ESLs are the exception.  I shopped for Martin Logans for years and never could get past the less than perfect integration.  The M/Ls have gotten somewhat better lately.

The JansZen's integrate the two driver types flawlessly.  Maybe it has something to do with the D'Appolito MTM design and/or its crossover approach, but the integration allows piano to play through the full range without any crossover or timbre change evident.