The Access Violation error seems to be an out of memory error, plain and simple.
After playing some more with the settings, softness is actually working pretty well when using Strength=1 and 2 or 3 passes.
This is the best result I got with Passes=3, Strength=1, Softness=.25
There should be further increase in quality once I get to do Bicubic downscaling via shader, as it will avoid clamping on the diff map.