@AcidWeb You're right and I was terribly wrong
I had to experiment and found out that, at least in my test manga, a value of 0.30 for Gamma gave me optimum results.
The problem is that the "Auto" setting produced very dark images 'out of the box', while KCCreator had nearly perfect results at first try.
Since I imagine that 0.30 gamma value cannot be the right one for every comic.. is there a way to 'tune' the Auto setting or improve it somehow?
Many thanks!