I like adding distortion to only high frequencies on vocals.
VX-64 allows you to choose low, mid and high, or a combination of all three, so I use it quite a bit. The Softybe Saturation knob is also great, as it can be switched between low or high. The Sturation Module can do something similar, IIRC., but I use it less frequently.
I know it's not what you're looking for, but I thought I would offer some alternatives.
HTH