I would like to know this too, I get those "dirty" shadows often. My setup is pretty similar to the one showed, only I use the Box as post filter (never bothered to change it) I use 3 lights in my standard setup, one with raytraced shadows (so I have raytracing set to ON) and 2 with shadows disabled.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.