Alright guys. I fixed the problem although I'm not quite sure what the issue was.
I pulled the strings AGAIN and went with string set number 4. Dings in stainless. I very deliberately installed them, exercising care to make sure there were no overlaps, twisting, etc.
I popped the plate off and wiggled wires. No Rice Krispies.
I checked for loose neck bolts, bridge screws, anything that could cause weird harmonics.
Pick ups have not moved and action if anything, is bit higher than I like. Still, I backed them down a hair. Like 1mm tops.
Plugged in and no more warble. Nothing but sweetness and that Dingwall tone.
Talk about losing my mind chasing a tone gremlin on a passive bass. I have never heard anything like that in my life. I was even looking for insane things like hairline cracks, etc.
Ok, I can stop clutching my chest. I've had this bass for a while now. And hundreds of play dates. You can count on it to be perfect every single time you plug it in. I've come to expect that.
Sincerest thanks to everyone for jumping on this to help me get it resolved. May none of you hear that God-forsaken tone!