I read Bob James's bio on All Music Guide with great interest (quoted below), and I think he was pretty avant-garde going back to his college days:
During the first semester of his sophomore year [at U of Michigan], he transferred to Berklee College of Music, in Boston, Massachusetts. James dug deep into free jazz with area musicians and formed his own trio... After graduating from Berklee, James moved to New York, where he met Bernard Stollman from ESP-Disk. He brought the impresario a tape he'd produced with his own trio and some classmates from his days in Ann Arbor, namely composers/sound artists Robert Ashley and Gordon Mumma from the Once Group.
Ashley and Mumma definitely qualify as avant-garde. Ashley is a personal favorite, composer of the epic (IMO) Perfect Lives.