Ken Caillat claims that the 70s-set play is 'substantially similar' to his memoir focused on the making of Rumours Ken Caillat, a producer known for sound engineering Fleetwood Mac, and Steven Stiefel, co-author of a memoir about the band, are suing the makers of award-winning musical Stereophonic. The widely acclaimed show, which picked up five Tony awards this year, focuses on a fictional, mixed-gender British Amer…