I think I'm the first guy to complain about the discolored knobs. My 500 head had them, and a replacement that Sam Ash offered me had them too. Fender stepped up and replaced the knobs, they were perfect. Then I put the amp in its little bag and it sat for awhile... and the discoloration is back.
Therefore, the problem is with the bag - the dye is somehow interacting with the plastic in the knob. I'd say rubbing off, but the discoloration won't wipe off.
So there, problem identified, but not actually solved.
But what's this I hear about new Rumble gear and a 2x10 cab? God, did you hear me at last?