I own a Thunderbird PRO-V (NO) aswell and found the hum unbearable. Even though I didn't try any of the mentioned methods, I found the solution by accident when I touched the potentiometers without the caps on, and found out that only when touching specific ones (volume and treble) the hum goes away, then I tried connecting those with a metal object (without me touching the object ofc.) and voila the hum was still gone. Then I just attached a new wire inside.
I soldered a new wire on the housing of the treble potentiometer (GND) and the other end to the housing of the volume pot. (GND) Eliminated the HUM entirely, and theres no cracking or whatsoever!!!
Just wanted to share my discovery with all you dissatisfied Thunderbird players in hope it works for you too. :)
here's the proof :)
