Jordan -
Heater winding for the 5U4 shorted to transformer case or other windings?
I would remove the rectifier from the socket and check the voltage(s) from socket pins 4 and 6 to the center tap of the transformer. Don't know what they should be on that particular set (maybe in the neighborhood of 400-450 VAC) but they should be close to each other.
Another thing you could try - check with an ohmmeter between B+ and center tap to see if anything is pulling the B+ down.
Hope this gives you somewhere to start - I know how much fun it is working on sets in unheated spaces...
Jim