Gregg Allman's most visible contribution to rock music was as lead singer, organist, and songwriter with the Allman Brothers Band, founded by his brother Duane in 1969. He never threatened to eclipse the band that carried his family name, but he found occasional success and popularity with his solo work, which was distinctly different, more soulful, and less focused on high-wattage virtuosity.
Allman's instrument was the organ, and he was most effective, when in top form, as a singer. His first instrument, ironically enough, was the guitar, and he took it up before his older...