THE BRITISH IMPROVISING GROUP AMM has had an impact as mysterious as it is evident on the world of experimental music. Formed in the mid-’60s by what founding member Keith Rowe called a group of “skinny white European street kids” influenced by African American free jazz, AMM grew out of series of workshops at London’s Royal College of Art, its early performances attended by the likes of Ornette Coleman, Paul McCartney, and Syd Barrett. Rowe, a musician and painter who’d played in Mike Westbrook’s big band, turned his guitar flat as Jackson Pollock had laid flat his canvas and also sneaked in