@roxy54 You nailed it. Massed violins. Far too often they sound like mush, even on costly systems. Especially on digital. It seems the digits have trouble resolving multiple HF sounds that are all playing the same tune. If you can hear individual instruments on your system then it's tops.
After that, solo piano. Does it sound like a piano? Too often, not entirely. However, poor piano sound is often down to the recording. Miking a piano is very difficult. Too many close mike. That is not what is required.
Those who said 'vocals' are incorrect. The human voice has tones in the middle, easy part of the range. Most systems reproduce them realistically.