The OED vouches for "Bachian." It would seem to be a relatively recent coinage, given how long people have been listening to Bach: the first citation the OED provides dates to 1920, and that citation uses "Bachian" as a noun; the first adjectival usage cited is dated 1945.

Curiously, "Glassian" does not yet appear to have entered the lexicon. I happen to enjoy much of Glass's work, although I know a few musicologists who would argue that there's no need to coin the term "Glassian" when the phrase "maddeningly repetitive" is more than sufficient.