I've actually seen a number of those heavy bands: Hendrix, and Vanilla Fudge at the same concert, ELP, and (from the 1969 Atlanta Pop Festival: https://en.wikipedia.org/wiki/Atlanta_International_Pop_Festival_(1969) ) Led Zeppelin, Johnny Winter, Joe Cocker, and Grand Funk Railroad (plus others).
Nothing shabby about Mountain, Wishbone Ash, and Black Sabbath--I just never got the chance to see them live.