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?
+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.)
For something different, visit a piano dealer. While someone playing, listen from 5 ft away. 15 ft away. 25 ft away. It will be educational.
Getting both vocals and a grand piano right would the ultimate success.