The main idea of this request is to refine a luminosity mask. The level or curves use is the first step. After that you will apply brush with blend modes like overlay to the combined mask and level or curve for get the right mask. With the smadell tip you can modify the luminosity of the mask using curves or levels, yes, but you lost the effect when try combine with the mask or you can't see what is happening if use a brush with a blend mode to refine the final mask. For now I load the mask (alpha channel) as grey pixel layer , apply curve or level, work with brush set as overlay to get the right mask and rasterize it like mask again.