I'm curious why you think that the bass hump isn't some resonant artifact of the measurement head that needs to be addressed in the compensation curve.
Here's my theory: It's due to the way that the sound waves impact the measurement head causing the air inside of it to resonate resulting in higher sound pressure on the measurement mic.
It is definitely centered around a harmonic of the big hump at ~3.5kHz.
I'm prolly just pulling things out of my ass though. I haven't reviewed my physics for some time now.