I think you are barking up the wrong tree I'm afraid!
Magnetic pickups have a high output impedance, and many vintage style fuzzes are designed to work well directly connected them. A buffer converts this to a low impedance output, which is better for driving long cables without signal loss, but doesn't play well with some fuzzes. The preamp in an active bass is essentially a buffer, so adding another one is not going help things.
What you need is a pickup simulator. There's not many off the shelf solutions to this that I know of, although I have seen some pedals (Seymour Duncan?) with a 'drag' control. Here is a circuit that does what you need:
http://www.muzique.com/lab/pickups.htm
I think BC member (Toasted?) made a few of these up a while back for people.
If anyone knows of a little mini pedal to do this job, I would be interested too! Actives don't play too well with my Team Awesome Fuzz Machine.