Interesting read!
One thing I would like to mention is: Ambient Occlusion is not a realistic effect! When used on a human head, you are darkening some parts of the ear that happen to be thin and those thin parts let light through from behind (subsurface scattering), so you are making something that has more light, darker…You can see that very clear on the ear of your head model. The SSS and AO are fighting each other.
Maybe I’m wrong and the AO is simply too strong in your case..
