I have had the same problems with my on plates not having enugh density in the black areas. Some of them did come out perfect though and tried to reproduce the effect but had no succes. I think the slightly fogged shadow areas are about as good as they are going to get. My next plan was to try Rockland emusion to see if that made any diference.
Like you I tested varying amounts of this and that but had no better results. I am wondering if Rockland uses something else in there developer additive.