Ok, well it was just an idea. If modifying the voxel code didn't work then writing a custom version of the LowPassfilter is probably the next logical step.

Glad you have a working solution anyway.

Are there some screenshots that demonstrate the before and after of this filtering you guys are talking about?

Sure.
This a result with the LowPassFilter.
On the left the density value is the maximum on the right side the same object after the LowPassFilter.
As you can see, the shadowline is not perfect. But if I run the LowPassFilter more often, the result gets better and better (also takes more time).