Thanks for answers.
Today I had the opportunity to try it with a line 6 helix and a friedman box. With the same result. My band members confirmed my impression. Normally I'm playing a Kemper and thought that's something with it.
If the mp3 of the JTV-69 is recorded direct and not thru an amp,....
==> Thru amp
-If it's strung for left handed playing.
==> No
-If the previous owner tampered with the way the piezos are wired
==> what do you mean?
-Distance of pick-ups to the strings inducing warbling.
==> looks like normal distance
-What is the Flash firmware version?
==> latest
-Intonation and tuning has to be spot on, or frequency artifacts can occur.
==> what do you mean?
-Being an earlier batch, it's probably long over due for a maintenance check.
==> is this recommended by line 6?