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

what design in your equipment should you seek to eliminate all of your issues?

Change all to most and I would say get a system that uses a minimum of a dedicated center channel and two subwoofers ( a 3.2 setup) instead of just two channels. Even Frank Sinatra used a center channel:

@edcyn   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?

steakster 

+1

Vocals are the most challenging. We hear human voices every day for a reference. Do they sound natural in your system? (Allowing for a slight touch of reverb.)

Big +1.

Vocal has to be the most challenging music to play on a stereo. Vocals in any audio system, regardless of cost, are far back away and empty void. I visited many audio shows hoping ultra expensive audio systems might be close to the original music. But nothing was really even similar to the real music. So, I made Wavetouch audio system that produces human voice sounds like the human voice.  https://www.youtube.com/watch?v=XaBy9CcGoTE

Alex/Wavetouch audio

@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.

+1 Kota

Iron maiden.  Rhyme of the Ancient Mariner, at full volume pushing to and maybe right up to over over 110db..