Not all the gospel music at Jazz Fest takes place in the Gospel Tent, a fact that will be brought home with great spirit by the outstanding vocal harmonies of the Holmes Brothers, who combine their gospel roots with rock soul/rhythm & blues grooves, a repertoire that ranges through rock, pop and country and a jam-band aesthetic that anticipated the “sacred steel” movement by a decade.

“I’ll bet you hear us do something you’ve never heard before at Jazz Fest,” promised drummer Willie “P...