EVA CASSIDY

Eva Cassidy was superb. Here are a few more great female voices that that I am enjoying lately: Vienna Teng, Alison Krauss, Holy Cole, Patricia Barber, Jenifer Warnes, Mary Black, and some may disagree, but even K.D. Lang's later works, e.g. her version of Hallelujah. If you dont mind French, try Carla Bruni also.

DCstep, you are probably right re Barber -- a little dark and different by comparison with Cassidy, but great range/voice as you know. The song showing of her skill would be "too rich for my blood". A great test for your hardware.
Some of Cole's work is a little lighter. I am taken with her song "Trust In Me" from the Jungle Book (if you know the movie context), and "I can see clearly now", which is a song that has nauseated me over the years, but is suddenly injected by Cole's magic.