This was one of my favorite songs of the late 60s:
I've never listened to it enough, though, so I've posted this live version from 1987 to remind myself. Linda Ronstadt did a decent version, which can be seen here. (I remember Lester Bangs saying in a review of Ronstadt's version that it was a good effort, but Mary Magdalene herself couldn't beat the original.) According to Wikipedia, Etta James also covered it; I must try to find that one. I'd like to hear Park Hyo Shin or Big Mama give it a try. I have always loved these big, throbbing, emotional voices, and songs of emotional meltdown that convey the experience as "Down So Low" does. (Or Eric Clapton's "Layla." Or "For a Thousand Days" by Lee Seunghwan -- sorry, the video is dreadful but it's a great song.)
I suppose one reason I haven't listened enough to Tracy Nelson over the years is that she herself never did anything else with the power of "Down So Low." It's a hard act to follow. I have several of her albums on vinyl, and while I always enjoyed listening to them when I thought to put them on, I never felt quite the compulsion to do so that I did with so many other people. But as this video shows, she's a great singer, still performing, still recording.
(Almost forgot -- h/t to Avedon.)