I would definitely disconnect the specular, I think it defaults to .4 or something which is the right thing to do. Roughness is also ideally a single channel but you seem to have greyscale in there so thats OK. Your Albedo should be much brighter, in a PBR situation you want the pure Albedo which is much brighter than you would think. I personally would have the rocks rougher too - brighter in the roughness mask