That's right, the Grateful Dead only played four shows in 1975, and they are ass-kickers. They just about went broke with the Wall of Sound in 1974.
Those are JBL speakers with big magnets, try dragging a Fender Twin with two of them around. They were moving serious air, crystal clear. This jam is a pippin; I'll repeat that I don't think there's anyone alive who is equal to Jerry.