Hi, @Schwiftaaay and also welcome!
Hmmm...I might be wrong, but I pretty sure that, while they are right that there is a strong enough signal from the piezos to make it perfectly possible to take a jack output straight into an amp or DAW without a pre-amp, nevertheless mixing that signal with the output from magnetic pickups is quite another thing.
To do that, while it isn't actually a pre-amp as such, you do need something electronically similar to balance the impedance difference of the two types of output. I fitted a Hannes Piezo bridge to this:
But I installed a Schaller Flagship blend unit in it to enable the two outputs to be balanced. There are a number of onboard blender/mixer units on the market but I think something like this is probably essential if you want both signals to be routed through a single jack.