Using Bump maps is interesting, when they show the hair or muscles or skin imperfections. It can be especially useful in high quality render engines like Cinema 4D. If they sell Bump Maps they should be correct IMHO. Inverting the bump map for the body should do it. It may be a bit more difficult with the bump map for the eye and the face. They did correct the eyebrow on the face map, so inverting won't help here. I wrote already 2 mails to Daz requesting help on this matter and no response til yet :-(.