Hi there!
I think maybe something can be messing with my plugin on your end. Here, I did manage to make it work with negative values.
So, you are using any plugin that messes with the parameters of the actors?
For example, my Class Curve plugin can set a minimum value for the actor to have on a chosen parameter.
Or maybe, what you are trying to say is, that you want to use the Dynamic Parameter Feature, to not just reduce a parameter, but also to make the actor himself, have a negative value. Is that it?
Like an actor with -10 atk. ?