So I bought one of these from CertainBass on ebay.
With my Ibenez Soundgear 480, I didn't notice any change in the tone.
I emailed Dave about this & he said
"sometimes they work funny with active electronics, sometimes it's OK. It's possible to install it in an active bass before the preamp circuitry. I have two protoypes of active varitone pedals that will handle any kind of input, and it's even strong enough to push a raw power amp."
He offered to take this varitone back & sell me a prototype for cost. Nice guy!
So I tried my TL Martinson through it, kick ass! The Martinson is passive and only has volume-tone knobs. Growling like a pissed off rottweiler!
I plan to get a blank rack panel & mount this on it so it can be in my rack permanently.