The old metal tubes could not handle the screen voltage and plate dissipation that a modern 6L6GC can. If your SSE runs at a lower than the usual 430+ volts of B+ maybe. It's possible that good tubes may live OK in a higher voltage SSE if the cathode resistor is increased to lower the dissipation to about 15 watts. I have run them in one of my amps, but I did not run them for long enough to see how then held up.
See this thread for more details:
Are 6L6 Metal tubes plug and play
See this thread for more details:
Are 6L6 Metal tubes plug and play