About 50 years ago Bud Fried(Irving M Fried of IMF speakers) told me he thought the 3 hardest things to reproduce in order are male voice(we are so familiar with it and it goes into an upper bass region speakers often suck on), female voice(again we're so familiar with it) and piano because it has such a huge dynamic ranger which is often very short duration demanding linear level changes that stop and start very quickly stressing a system and especially speakers hugely.
Bud used the term dynamic linearity. He didn't just mean the ability to play clean and loud. He meant the ability to accurately follow all level changes, small to medium to large. I find this to often be the achilles heel of systems that otherwise sound clean and tonally accurate, the difference between a wonderful wide band radio and something that at times lets you make believe it's real. My main source for this is a Kissin piano 'Pictures At An Exhibition,.