Vortex Sound Research

Looking for a low gain overdrive to add just a bit of grit and warmth to a digital synth. Something thats basically "transparent" but maybe rolls off the highs a little tiny bit. I prefer passive EQs over active since those seem to leave the core frequency of the instruments alone.

I've tried a bd-2w (too scratchy), operation overlord nano (changes the tone too much), and a boss od3. Currently using a DOD bifet boost which does add some warmth but not any grit. Any suggestions?