Minimal heights and defect groups with two character degrees

Conjecture A of 3 predicts the equality between the smallest positive height of the irreducible characters in a p-block of a finite group and the smallest positive height of the irreducible characters in its defect group. Hence, it can be seen as a generalization of Brauer's famous height zero conjecture. One inequality was shown to be a consequence of Dade's Projective Conjecture. We prove the other, less well understood, inequality for principal blocks when the defect group has two character degrees.