Take into account, older hair was subjected to many Poser 'hackery' to make them look decent.  Baked-in specular/highlights, generating bump/displacement maps with math nodes from the original Diffuse texture, and active ambient channels were some of the techniques employed in the material room.  Shortcuts to simplify the production process and get it to market quickly.  

I do agree, there are significant differences between lighter-color and darker-color hair.  Darker hair requires more detail, and the contrast should be exaggerated so that you see a difference between the strands.  You either use math in the Material Room to accomplish this or take the maps to a third-party program for more manipulation.