VERY astute observation, Ed.
Using just tube filament currents ( per tube data sheets )...total = 4.7 amps.
Power = 4.7 x 6.3 = 30 watts
30 watts @ 120 vac = 0.25 ac amps
10 ohm voltage drop = 0.25 ac amps x 10 ohms = 2.5 vac voltage drop.
120 vac - 2.5 vac = 117.5 volts AC to rig xmfr. = STILL TOO HIGH with rig in "idle".
Your idea of using dropping resistors seems DOABLE for only $2.63 each leg...size is 1 x 0.4 inches:
7 - 6.3 = 0.7 voltage drop
resistor value = 0.7v / 4.7amps = 0.15 ohms
resistor wattage = 3.5 watts
Please see this one:
https://www.mouser.com/ProductDetail/TE-Connectivity-Holsworthy/SMW7R15JT?qs=zW32dvEIR3tzutB0RL4gyw%3D%3DI also used surface mount caps to fix silver mica disease in IF cans.
Next...getting that 400+ B+ to proper value.
My suggestion:
1) Measure Key Up voltage at F and F,
2) Add up all filament currents from Franks Tubes, we'll call that Ift,
3) (Key Up Filament Voltage - 6.3 Volts)/ Ift = Rdrop,
4. Place Rdrop/2 = RLeg in each filament supply leg.
For example: Let's say Key Up voltage = 6.8Vac.
My total filament current = 4.5A.
My preferred filament voltage = 6.3V.
Rdrop needed = 0.5Vac/4.5A = 0.1 ohms
RLeg = 0.05 ohms for each leg from F,F, at 2W each.
Phil