I too love Diana Krall she is an excellent jazz singer. However vocally she is not in the class of Cassandra Wilson, Sarah Vaughan, Nina Simone, Rachelle Ferrell, Billie Holiday or even a Norah Jones. But again very few are. When I listen to DK she conveys her style and she engages you as few can. I again truly enjoy her music. However she vocally are not in the class of the female vocalist mentioned above. If you need reference material please listen to the following and let your ears decide.
Nina Simone - Pastel Blues - Strange Fruit
Nina Simone - Four Women
Sarah Vaughan - George Gershwin Song Book - Misty
Cassandra Wilson - New Moon Daughter - Strange Fruit
Rachelle Ferrell - Individuality (Can I Be Me?) - I can Explain
Cheers