Gen 6:4, Dept.
Listening to a stream of Miles Davis' sextet on April 12, 1970 at the Fillmore West. Jesus Mary and Joseph O'Leary, they started with an "It's About That Time" that must've flash-fried every hippie in that temple to Euterpe, then continued one minute voodoo funk the next free blowing for the entirety of the set, with occasional oases of quiet lyricism. If you've ever asked yourself as I am wont to do, "What's a good example of a band that listens to each other with elephant ears, then plays as tightly as, I don't know, some 12-armed deity of asskicking ecstacy music?," this will answer your question.
Then, the Grateful Dead walked on to headline, reportedly awed by Davis and his band. No offense to the Dead, and on their home court yet, but you can only follow an opener like that, to quote Lenny Bruce, with Art Baker whacking it in Bert Parks' face.