One of the most difficult to reproduce bits of music is massed strings, whether they be cellos, violas, or violins. It takes a complete and near perfect chain to get these right and they represent to me the biggest challenge and what I always turn to for the "final test" when designing product.
Power supply and feedback levels seem to be the biggest culprits in analog, while power supply and jitter appear to be the biggest offenders in digital.
Massed strings are some of the most complex musical waveforms I can think of, far more than massed voices. I imagine the spectral content of these hundreds of simultaneously vibrating strings is among the most complex of all and one of the areas where digital audio seems to really struggle with.
To be fair, speakers have a tough time with these as well. Vinyl appears to fare better in the area of massed instruments but, I suspect, only because the upper harmonics complexities are somewhat rolled off relative to digital audio.
My favorites for testing are the older RCA and Mercury recordings; not because they are better but because they are more difficult to get right. Modern digital recordings of massed strings are mostly better so they don't work as well for evaluating equipment in my experience.
If you can get these old masters to sound right, to get to the point where you can hear each and every instrument in its own independent state, then you've really got something worth listening to.
The rest is easy from there.