I think this would be interesting for you too. That's why I post this thread also on the pickups & electronics forum.
Since some time I was looking for an alternative electronics to put in my trb6p without loosing the piezo pup's. Finally I got in contact with a german engineer named Andreas Richter, who is providing preamp electronics amongst others to BassLab. After some discussions with him, I was ready to try his preamps (BassXX, and Piezo preamp, you'll find his products here
Richter-ee.de) and to put them in my tr6p. They came prewired and almost ready to use, I had just to connect the magnetic and piezo pup's and the output jack. The configuration I've chosen was volume, balance magnetic pup's, mixer magentic pups with piezo, stacked bass/treble, and stacked parametric mids and mid boost/cut. As you can see on the pictures it's possible either to adjust the gain of each piezo pup or/and the gain of the piezo preamp and there's a trimpot on the pup's mixer to mix the two signals adeqately. I have to admit, that I was very surprised by this new electronics where it's even possible to play with the piezo pup's allone and getting a nice sounding tone out of it, not to compare with the stock electronics.
This pictures will give you an idea (blame me for the cable salad...)