Piezo Pre, Pick Ups and impedance matching

[Max]

I almost finished a solid body with three single coils and electronics more or less like a strat.
As an experiment, I added a piezo transducer with a little preamp built around an MPF102. I have another preamp like this in my acoustic and works very well. In the acoustic, however, the piezo is under the bridge saddles and has a higher output, while here I have a single piezo element (aka buzzer) somewhere stuck to the body (well, under the neck pick up). First thing, I'll try to replace the jfet with a j201 to have a bit more gain and try to balance the output a little bit wrt the magnetic pick ups.
The main issue, however, is that I used an ON-Z-ON toggle switch, so that I can have both magnetic and piezo circuits together but there's some impedance problem and the result is that when they are in parallel the pick ups are basically inaudible. If I remove the battery, the pick ups can be heard as a far far away sound.
Here's the schematic:
https://drive.google.com/open?id=0B5Uj9BTxpfvLR3VwR2dkaFFTbnc

Any idea? I'd like to leave the magnetic pick ups passive...


EDIT: I just realised that I open this thread in the wrong forum, can someone please move it to the proper place, like "Open Discussion"? Thanks

[Max]

I've replaced the fet with a J201, R2 with a 2.2k and the trimmer with a 25k to get a correct biasing. This has increased the output impedance and the magnetic pick ups are a little bit more audible in the combined mode, but it could be much better. On the other hand I have to play a little bit with the biasing because the piezo alone now is louder than the pickups alone.
To be continued...

[Max]

It turned out that to balance the piezo alone wrt the pick ups alone I had to replace the fet with a 2n5457.
The acoustic sound is also less "boxy" (is it a real adjective???) but the parallel still doesn't work, though a slight change in the piezo is audible.
I'm thinking about removing the ON-Z-ON switch and using a push-pull potentiometer instead, removing the parallel option.

By the way, the tone in the schematic works very well for the magnetic pick ups but not at all for the piezo, any suggestions? I'm planning to try some different cap, I can't add another potentiometer or replace the existing one. I discovered that my old acoustic guitar had no capacitor on the tone control, I'm wondering how it was working.