[snowman_al]

That is very encouraging and it all falls neatly into place.

24 volts for the pre amp valve heaters is spot on.
0.035A times the HT of 391 volts is 13.7 watts dissipation, that is fine.
(I assume the 4 valves are all about the same voltage across the 10Ω resistors and are well matched.)

If you did want to reduce the dissipation a little more, then increasing R34 (2.2kΩ) to 2.5kΩ or 2.7kΩ will give you more negative bias...

Alan

PS: Should also mention that it is highly recommended to add a 100Ω 1 Watt resistor to each screen grid (pins 1/7) of the 7868s. Screen grid stability resistors, from the Gillespie articles.