[Amps] tube replacement ?

[Posselt Werner]

Hello!

I have an amplifier with 2x 4-400 which has a really big power supply and a
very rugged design.
The only thing I am missing is the appropriate output power.
This lead to the idea to replace the tubes by something different.
The two 4-400 are run as triodes in parallel.
So what has to be considered when changing the tubes to let's say 2x GS31b ?

First thought:
-Filament Voltage
-Input/output/feedthrough capacity.  Will it be necessary to modify the
output circuitry ?

Do you think this is worth a try or should I keep this almost
'undestroyable' amp as it is ?

73, Werner