I fixed a wobbly tone knob on my JTV 69 and want to let all know what I did and what not to do.
Do not - remove the Philips head screws holding the pick guard to the guitar body. There is nothing you want to see in there (trust me).
Do - obtain a super flat (stamped from sheet metal) crescent wrench of the correct size (unsure of the size - cause I don't have the right size wrench myself). Place masking tape on either side of the wrench if you are concerned about scratching the pick guard. Position the wrench between the pick guard and bottom edge of the knob and tighten the nut.
This should restore stability to the knob.
For what its worth Line 6 - you could send someone to Korea to check on the quality issues with the guitars - I think the price point of the guitar warrants the extra cost of eliminating poor quality, unhappy customers, and unnecessary returns. Despite my bitching - nothing catastrophic has yet to be discovered but one wonders what troubles lay ahead...
So far:
Missing allen wrench for whammy bar set screw
Model knob cap just fell off - had to be reglued
Model knob will turn on/off only in the Custom 1 or 2 positions
Wobbly Tone knob
Intonation issue with high e string
Just saying.