Reading this thread made me play my Portamento tonight.
I have a reduced volume setting you might want to try.
Turn both magnetic pups to full, then hit a note.
Then roll back the neck pup slightly until you hear a change in tone, then play that same note. It will sound different. Now leave the neck volume where it is, and roll back the bridge volume slightly until you hear a slight change in tone. Both magnetic volume controls should be at about 8 if the controls had numbers on them.
Play with both pickups at about 8; it should sound like both pickups at 10, but with a mellower tone.
Then, leave one volume at 8 and move the other volume control from 10 to where ever you want, always listening for tonal changes. Repeat with the other pickup. Take at least 10 minutes experimenting with this, and report back and tell us if you found a tone you like, but had never heard before.
I tried mixing the piezos with either the neck or bridge pickup, but decided I like the piezos alone.
Good luck.
Mike