I noticed that the high notes of the harmonics bars sound louder that the low notes. Is there a way to set the volume of the note played according to the pressure you play the note (touch sensitivity)?

There is no touch sensitivity in harmonic bars because original Hammond organ doesn't have.
Frankly speaking, it doesn't seem to me that high notes are unbalanced in comparison with low notes.
Do you have any high frequency enhancement in your effects settings or external audio system?