Done....
I used a different approach this time.
I smoothed the plot by 1 whole octave which obviously removed 'the grain'.
The headphone suddenly sounds very smooth....


Of course I had to lower the absurd amount of bass a bit and chose 'Paint' instead of filtering or damping.
I grabbed the bass area and 'compressed' it by a factor 3.
Much easier and the result is great.
Now the bass suddenly is tight and extends deep.
The channel imbalanced seems improved as well.
I am real happy with this plot.... I wish I discovered this sooner, would have saved me a lot of time tinkering with those things.