A similar point to Shirley's was made at The National (UK festival) by Jeff Davis (IIRC) in an American context (or in a general contect but with American examples). That you needed to go back to the source singers and learn what they were doing vocally to understand the possibilities (If I understood correctly, he favoured trying to learn as exact an imitation of the singers as you could, in order to appreciate the details of the style. This is pretty much how jazz players learn their art too; there are stories of those who insisted you be able to sing the improvisation of earlier masters before being allowed to play them on your instrument).

Mick