"Will the Powerchip allow me to blend my Powerbridge and magnetic signals onboard the guitar?
The answer to this is yes and no. The function of the Powerchip is to buffer the Powerbridge signal to better match up with the magnetic output. This impedance matching allows the Powerbridge and magnetic signals to be mixed to a mono or stereo output depending on the cable used. A mono cable will give you a combined piezo/magnetic signal to the tip of the output jack and a stereo cable will give you the magnetic signal on the tip and the Powerbridge on the ring. The Powerchip is not a blend pot, but a volume pot for the Powerbridge only. To adjust the amount of blended Powerbridge and magnetic signals, the Powerchip and the original magnetic volume controls must both be used."
So, it seems that the Powerchip is an impedance matching unit (what a buffer does) and also has a trim pot to balance (not a typical balance pot, tho) volumes. Just came across this- it should not need an additional buffer.
Link for info, 5th subject down: http://www.fishman.com/tech-tips