Yes, i can confirm some models look too dark, i.e. Austrian Pine from Europe 1. Since XFrog is a generic library (not specificaly designed for Max), i think the rendering result may change drastically depending of the platform.
I've tried several settings, and i got the best result using a multiplier at Bitmap->RGB Level (i.e. 5). If that helps, we would include such multiplier to the Material Optimizer. Obviously, we cannot know what textures would require one or other value, so is responsibility of the user to test different settings, to get the best result. I'm afraid we cannot do much more in this case.