The tritone was seen as 'diabolus in musica' in the middle ages, long before the advent of tonal harmony, because of the difficulty in singing tritone intervals, which made it a step that it was considered essential to avoid in linear counterpoint.
The view that the composers subject is eliminated under dodecaphonic tonal organisation is erroneous, in fact it's less prescriptive than tonal harmony, and certainly less so than species counterpoint.
It's been alleged that the Devil in Doctor Faustus is Schoenberg, but I'm not so sure. Although Schoenberg's music was at odds with the early Adorno's personal musical taste, his championing of Schoenberg's compositional method against Stravinsky's (and the fact that he studies with Schoenberg's most famous pupil Alban Berg) suggests that it wasn't as simple as all that. Given the book's anti-liberal slant, Mahler (the backward-looking, johnny-come-lately conductor/composer) is a more likely target for parody - although I confess I've never looked into it in detail. There is however a music critic character (another incarnation of the devil I think) that's probably a fairly affectionate thumbnail parody of Adorno.