What does it take to be a college professor in the music theory category?

Answer 1

The absolute minimum would be a master's degree with 18 graduate hours in music theory.

The preferred (and probably the only way to get the job) would be an earned doctorate in music theory. This would be a PhD, DMus, DMA, or DA (not a DME or EdD).

Many lower level music theory classes are taught by graduate assistants and by faculty from other specialties. Sometimes theory is taught by a professor whose main area is something entirely different (such as conducting or choir) but for a top level theory job, the doctorate in theory is the degree to have.