What is the most challenging music to play on a stereo?

Vocalists. Any genre. Do the male voices suffer from a buzzy midrange distortion? Or do they sound rich, clear and authentically human?  Are the sibilants spitty or hashy?  Or are they just like you hear when somebody's talking to you? The same goes for saxophones.

Big Band Jazz and Romantic Era Orchestral music can be good tests, as well. Big Band can test the system's ability to punch. Classical has a good chance of showing off a system's ability to create spaciousness and a believable soundstage.

@pangeek --

Yes, all the hammers on a true acoustic piano are padded. Some brands more than others. As the piano is played and the hammers strike the strings, they eventually compress & harden up and produce a brighter tone. When things go a little too far, a piano tech might soften the hammers, giving them a bit of their previously rounded tone.

I never owned a Steinway (I just never had the lucre), but the ones I lovingly "tested" at the piano dealers varied immensely, from bright to mellow. I have to say, though, that they never sounded rinky-dink.