Yeah, the sampler not intuitive at all, and lacking some needed, even basic, features. The sampler really needs some work. Badly.
... I hope they're on top of this, or at least listening to us; somewhere, somehow.
But no. There appears to be no way to adjust the threshold. Nor can I personally tell any difference between Velocity and Threshold modes.
I swear I read about being able to do so in the manual, but looking thru it just now I can see nothing about that. Perhaps I'm thinking of the manual for the previous version, Spark (which, in some ways, appears to be "better" than Spark2. I've seriously considered installing it, as I have the disc that came with my CDM)