Speaking of the Allman Brothers, especially Duane Allman, and the Fillmore East, they dropped by to see Jerry and the boys one night long ago. BTW, Peter Green (originator of Fleetwood Mac) was there too, and Jerry invited them to plug in later in the second set and this was the result ...
Grateful Dead Live at Fillmore East on 1970-02-11
Pay particular attention to the hour long jam, Dark Star, Spanish Jam, and Turn On Your Lovelight where Peter Green, Danny Kirwan, Mick Fleetwood, Duane Allman, Greg Allman, Butch Trucks, and Barry Oakley participated.