For me the only harmony singers that I would walk over broken glass to see and hear are Coope Boyes and Simpson. Their precision diction and excellence are unbeatable - I was even able to introduce a member of the choral society that I sing with in my other life to them without having to worry about their reactions. A barbershop singer that I know was knocked out by their technique and "over-ring" (not sure if that is the correct term - in other words production of an apparent fourth harmony for a three part harmony group). Can't get to see them often enough!