Iray Sun-Sky Implementation
It was a long time since I last used Iray Sun-Sky and now I wonder at a certain detail, the SS sun node (from scene).
The Daz documentation says:
"SS Sun Node - Provides the ability to choose a node in the scene to indicate the direction from which the sun is shining; the direction of light is calculated between the position of the chosen node and the center of the environment dome."
Testing this with two spotligths, one close, one far, both pointing full frontal at her face.
So the one far away results in a smaller shadow, which makes sense when it's pointing to x/y/z 0.0.0 and it's just about the angle, but the light seems to have a different temperature and intensity too, maybe some partial covering of the virtual sun by the virtual ground ? 
And would't it make more sense, when it would be implemented like the direction of the virtual sun goes along the direction of the scene node (or at least just pointing to x/z 0.0, no idea about possible implications, just asking) ?

Comments
Thanks for your answer, "Physically Scaled Sun" seems to work (for the temparature and intensity), and I would expect "Sun-Sky Only" not to be influenced by scene lights, but 've tested it, just in case.
Seems that the nice and bright sun shine effect is gone with PSS switched off