Possible Clues:

• Male singing voices
• Some of the knights in Wagner's 'Tannhäuser'
• Roy Orbison and Marvin Gaye, e.g.
• Choir section ... and what are missing from the starred clues
• Pavarotti et al.
• Singers above baritones
• Courses of thought
• Or x X invoiced?
• Domingo and others
• Choir males