After noting a couple of pages back that some birds don't react to tone knob adjustments as linear as you'd expect and that's not a function of a broken tone knob (as the two non bird guys on the video claimed) I decided to do a little test.
I plugged each of my birds into my amp, set the neck volume on full, bridge volume off, and the tone on full then played open GDAE strings finger style directly over the neck pickup while adjusting the tone knob down in one step increments. The results were interesting....
Eppi VP - no noticeable tone change from 10 to 4, then noticeably darker from 3 to 0
Gibby Anniversary - noticeable darkening at each step from 10 to 6, then from 5 to 0 no change
Greco - no noticeable tone change from 10 to 8, noticeable darkening with each step from 7 to 0
BaCH - no noticeable tone change from 10 to 4, noticable darkening with each step from 3 to 0
2015 - no noticeable tone change from 10 to 6, darker at 5, no change from 4 to 0
Conclusion - one could easily assume the VP tone pot isn't functioning IF the volume of a soloed pup is set at max and the tone pot is only rotated between 3 and 10. The Greco had by far the most linear tone pot and the 2015 had by far the least linear tone pot when volume is at max, all tone pots functioned just fine when the soloed pup volume was at 8 or less.
Something else to consider - I have all my pups raised to the max and strings nice and low, so I may be experiencing a bit of natural overdrive or over saturation in the signal that could be impacting the tone.
If you wish to repeat this test on your own bird it's very important that the volume of the soloed pot is at max. Anything less than max gives completely different and mostly very linear results. I also suspect that using a pick would give different results.