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Tubes redplate when excessive current flows through them. The only flyback failure that could cause excessive current through the HV rectifier tube would be a short/breakdown of the rectifier filament winding (the 1-2 turn loop around the core).
Try running the set briefly with the anode lead disconnected from the CRT, and the 6BK4 pulled out. Measure the HV at the anode lead. The failure HAS TO be downstream of the HV rectifier. Fortunately, there isn't all that much circuitry there. The excessive current through the tube is going to ground :
A.) Right at the rectifier filament because of a carbon tracked socket/mounting cup or a failed filament winding on the flyback.
B.) Through the 6BK4, because of a bad tube, or a failure in the HV regulator circuitry that is turning the tube on hard in an attempt to pull the HV down.
C.) Through the CRT, due to gas or an internal short between the internal dag coating and some part of the gun.
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