I don't see any female vocals on your list so I'll make a suggestion.
Jane Siberry -- When I Was a Boy. The track titled Calling all Angels is a duet with kd Lang. Because their voices are somehat similar, if I can hear two distinct voices singing side by side then I know things are right.
For male vocals I like The Nitty Gritty Dirt Bands, Will the Circle be Unbroken II. First cut is Johnny Cash, followed by John Prine, followed by Levon Helm, followed by Ricky Scaggs followed by John Denver.... you get the idea.
Just two for you.