I would suggest Melda's
MCharacter, which is the only thing I know of to surgically modify harmonics in real time. An EQ is not capable of this precision, and you may be shocked by how much slight harmonic changes will affect the overall timbre. The teaser has a specific comment in it about "harsh/nasal" sound in a vocal.
If you have never run the MCharacter trial before, you could simply use the trial for your work now;
but be sure to bake in the results so you will have that copy after the trial expires.
Edit: Forgot to mention, use MCharacter on the dry signal
before any further processing to get the maximum benefit from it!