377v isn't bad for B+, 366v at the 6V6 plate (pin 3) is fine.
"One other thing I noticed is that the resistance measured over each 100 Ohm cathode bias resistor on the V3 and V4 sockets measures 50 Ohm. I looked at the schematics, and they are in series, so I don't quite understand that." The cathode bias resistor for the power tubes is the large 250ohm resistor on the board that connects to both power tubes pin 8s. If these 100ohm resistors connect to pin 2 or 7, then they are the heater virtual centre tap. The junction of the 100ohms may be grounded, or connect to the hot side of the aforementioned 250ohm resistor (6V6 pin 8). Either way 50ohms seems fairly reasonable as a reading.
The resistance between the 2 filament leads will read "0ohms", or lowest default setting on the meter because of the very low resistance of the heater winding of the PT.