Who is this guy?
A 13th-generation American of all-American mongrel stock. A second-generation red-diaper baby, named after Karl Marx and Frederick Engels. An ex-Leftist, recovering spasmo, and incurable brainiac. A déclassé intellectual settling into crotchety-old-bachelorhood, whose main interest in life is watching the science-fictional future turn into the historical past.
When you consider how the interjections "ah" and "oh" are pronounced, the pronunciation of Jahn is far more obvious and logical than that of John.