It's quite strange to me that classical music has fallen so far out of favor with audiophiles as music to evaluate a system, or a new piece of gear.
For me, classical music is overall the best for evaluating a system.
Classical music is recorded: with all the musicians playing at the same time, in the same acoustic space, where the recording engineer endeavors to capture an accurate representation of that event. Including: the acoustic space itself, the musicians position within it, their position with relation to each other, the ambience, etc.
When a musician seems like they are to the right or the left of another musician, or further back in the soundstage, it is because that is where they were when the recording was mad. Not because the studio engineer took their individual mono track, then panned them to that location.
It is a soundstage and imaging that has a referent to an actual event, not fabricated in a studio. It is a qualitative difference.
Also, since the instruments are recorded with a minimum (or no) of effects done (digital or analog delay, reverb, echo, phasing, quantization, etc, etc), the sound of the instruments are going to be much closer to how they sounded when the recording was made.