I am the last person in the world who can judge the quality of a singing voice, but there's an African-American man who sings bass, who sings opera in the Montgomery Street BART some afternoons when I'm leaving work. I love his voice and always give him a dollar.

(The buskers in the Montgomery Street BART clearly have some kind of agreement; there are maybe 5-8 morning ones and somewhat fewer evening ones, but they clearly take turns and don't trespass on each other's hours. One morning group is a teenage (or maybe college-age) string quartet.)