I use difficult music such as Sarah McCoy's "Blood Siren." It's a mysterious and unorthodox album that I love. McCoy has a speech impediment and a cajun accent to boot. When I ran it through recently acquired Audion mk3 monoblocs it was when I was hearing lyrics previously unintelligible that I knew it was broken in and measuring up to the hype.
Also, "Harrow and Harvest" by Gillian Welch. Her voice can be treble sibilant and irritating in a lesser system. Th Audions tamed it without killing any detail.
Also, James Taylor's "October Road." Taylor's voice can be honky in less refined systems. A grown system tames the honk while preserving all the details of this masterful recording.
Also, Leonard Cohen's album "You Want It Darker." In an immature system Cohen's voice can be unctuous and gravelly. In a mature system all of that disappears, and his voice becomes a fine instrument floating in a bath of fine instrumental balance and angelic chorus.
On all of these albums if the midrange is not engaging then the emotive power of the human voice is absent as well.