You are right about the 270FX - I see 290-295V on the HT
secondary with a line voltage of 117-118V. With a 5AR4,
this means 365-370VDC which is a bit much for the EL84's.
A workable compromise is to use the Sovtek version of the
5Y3. It is not a direct copy of the original - it uses an
indirectly heated cathode and drops much less voltage. With
the 270FX, I get around 345VDC which is a little kinder to
the 84's. The amp still has decent transient response (the
NOS 5Y3 has too much sag for my tastes) and the tubes
seem to be happier. FWIW....