My first instinct was right, the rectifier tube was badly or wrongly seated. Pin 8 was mangled, the clincher thingy inside crushed. Has that ever happened to anyone before?
When I tested the tube on my trusty Eico Demonic 666, the first half tested good, the second half test resulted in a lovely flash. So that's why the second explosion occured when I powered up after replacing the melted resistor.
Stole another PC octal out of my half-finished Aikido and awaaay we will go, I hope.