Cassandra Wilson - New Moon Daughter - eclectic, haunting vocal jazz - but it may be an acquired taste. Certainly not soft contemporary or pop. I heard it used to demo a tube amp / LS3/5a system and it was incredible. She covers U2, Sam House, Hoagy Carmichael, Hank Williams and Niel Young on that album. I ended up buying several of her albums.
Since you mentioned bluegrass, you can't do without a little Allison Krause. I like the "Forget about It" album and it is my go-to album to listen to a new piece of equipment.
Another Jazz vocalist I like is Joan Armatrading. I had her "Walk under Ladders" album on vinyl and it was one of my reference albums back in the day.
I have a greatest hits album of hers. Perhaps not the best production overall but there are a few nuggets like "Love and Affection" and "Willow". These are all circa 1977 - 1986 so the recordings reflect the equipment and style of the times.
Browse through the Thread on "Songs that make your Headphones WOOOOW" and you will get lots of recommendations.