I guess one of the other dimensions of voice quality that might enter into this is race/culture. A woman I loaned my Eva Cassidy CD to said if she hadn't read the liner notes she would have thought Eva was a black woman (although I don't hear it). When Elvis Presley hit the scene, his manager, the Colonel, reportedly expected great things because he had found "a white man who sings like a black man."

I hear something in the voice of Nana Mouskouri that I associate with Greek singers, and,recently, in the film Vengo, by gypsy director Tony Gatlif, there was a Spanish gypsy singer who had a powerful, raw, primal sound that just bowled me over. I also hear that same raw, but slightly different, sound in the voice of Hazel Dickens, the American Appalachian singer.