Re: JPMM piezo system
You can plug into either jack to play magnetic pups, but only the mono jack allows you to use either the pickups or the piezo through the same signal. The stereo jack will accept a stereo plug which you can split later to send one output to an amp and one to a PA, for example. The mono output jack allows you to run the pickups and piezo. You just can't split the signals. I like running mono out if I'm just jamming around and I've never really had the need to do the stereo thing.
The top selector switch on the upper horn controls the piezo. In the up position, it's just piezo. In the middle, it's piezo and magnetic. In the bottom position, it's off completely. Your 3 way switch on the bottom of the guitar is for the neck position humbucker, the inner coils of the humbuckers, and then the bridge humbucker. Keep in mind that if you use the piezo and leave the guitar cable plugged in you can drain your battery and that will make the guitar sound whacky.
Enjoy! It's definitely a great sounding guitar.