So it makes it sound that implementation of a separate CombineRGB node, where P13 is concerned, is incorrect and that the Subsurface Radius values need to be inputted directly.  Also, there is a random walk and random walk fixed radius features under the SSS Method contextual menu, which one should be used?  

In addition, do underlying values, like the Surface Radius, become part of the multiplier calculations or do connector nodes override those values?