Also Sarah McLachlan (The Freedom Sessions & Surfacing), Jane Siberry (When I Was A Boy) and Lisa Gerrard (Duality & Immortal Memory). The common thread in these singers is their purity and the ability to seemingly just open their mouths and the sound rushes out (in contrast to someone like Celine Dion, who, after about mid-volume, always sounds like she's straining, to my ears).
Good luck & have fun! Hey, maybe report back on your search!