Re: Bass distortion effect
this tends to be how distortion units work anyway, they are gain dependent, but most do have a gradual curve that starts almost straight away.
anything with a spline graph will allow you to set up your own shapes and threshold with ease. personally i love izotopes trash 2, but they all do pretty much the same thing.
another way of adjusting the threshold would be to adjust the input and output gain inversely proportionally, so you could send both inputs to a gain plugin (if needed), then through the distortion, then to another gain plugin (again, if needed), then lower the input gain and raise the output gain (by the same amount) to make the threshold higher, and vice versa to make the threshold lower. this is pretty much what a threshold control does on all effects that have one. usually it only adjusts the input gain on the sidechain (internal) signal though, but the result would be the same, with slightly more CPU use.