I somewhat agree with the OP.
When did it happen, that the use of the term, "female vocals" has almost become a genre in the audiophile world?
I love female vocals, but it has become almost synonymous with a sort of 'not quite jazz', lightweight, "safe" form of music. It does not actually mean, serious jazz, classical, avant-garde, female fronted progressive bands*, or even female fronted symphonic-metal bands.
But I disagree with Tool or Opeth being being used for demo purposes. As big of a fan I am of both bands, they are not good for evaluating audio gear.
Just because a form of music is intense, and played loud, does not mean it is hard for audio gear to reproduce.
When it comes down to it, the best recordings for evaluating audio, are recordings of acoustic music, where all the musicians are playing at the same time, in the same acoustic space. With very limited done to the recording after the fact.
It is harder to accurately reproduce acoustic instruments and vocals, than the vast majority of mainstream studio recorded, rock recordings. And it is easier to hear if a system is accurate with acoustic instruments and vocals than electronic instruments.
*Listen to the band Renaissance, with Annie Haslam and her 5+ octave voice, with near perfect intonation, and power to spare.