I never know exactly how the different sound programs are going to react with each other. So far, even when having two mixers installed, I have not created any real disasters.
What I have found is that when I am not using pulse audio, Qasmixer has always been the solution to getting a sound function working that kmix does not show.
I just looked on this machine and under qasmixer, there are separate sliders for boosting the front and back microphones.
The range on those sliders is -30db to +30db.