EBMM Sterling Piezo 4H – two questions for experienced owners

First of all happy to join this forum! This is my first post.

I recently purchased a EBMM Sterling 4H (2005 model – F12470) including Piezo. I am a proud owner and I would like to learn more.

Q1.
The three way switch (serial,single coil,parallel) IMHO does not seem to do much. When turning down the piezo element it tends to offer some more variety but my overall impression is that the tonal differences between the 3 settings are hardly noticeable. Is this a correct statement or could there be a faulty switch?

Q2.
On the back there is a cover plate. When removed it reveals a number of “potentiometers” which allow tweaking / configuring using a small screwdriver. Is there an instruction manual what these screws exactly are for and how to adjust safely? I would imagine these meters are used to tweak the sound.

Thank you all for your help!
Ray

The Netherlands