McMartin guided the career of Hoodoo Gurus for four decades and established the professional framework for the music management community.
The Grammy-winning musician performed frequently at La Scala opera house in Milan.
The Oxnard, Calif., native's manager, Jack Sills, announced the news on Instagram.
The rare student of Vladimir Horowitz, he made a dazzling Carnegie Hall debut at age 20 and performed major concertos from Chopin, Mozart, Rachmaninoff, Liszt and Prokofiev.
The British singer-songwriter was undergoing cancer treatments.
"The Band will miss our friend, brother, and band mate Anthony 'Baby Gap' Walker," reads a post. "Gone too soon. We will never forget you!!!"
Boyd joined the Academy as an executive secretary in 1968, as the organization's first paid employee, and rose through the ranks.
"It brought him great joy to know, that for decades, his music touched so many and will be his lasting legacy," his wife Amy Carmen writes.
Power built the Mean Fiddler Music Group from the ground up, an empire that began as a single venue and grew to one encompassing 30 venues and events.
The vocalist was also known for singing with Manilow, Bob Dylan and Mary J. Blige.
