If a diode style rectifier is used for the 5AR4 then it would free up the 5 volt 4 amp 5AR4 heater. If this was run to a voltage doubler circuit you would have 10 volts at 2 amps. Then run to a voltage regulator to lower the voltage to 6 volts. Then use this regulated 6 volt DC to run the driver board heaters. If 10 volts is too high you can add a resistor and another capacitor before the regulator. Maximum required parts I would think would be 4-1N4007 diodes, 4 electrolytic caps. 1 resistor, 1 regulator with heat sink and a small board to mount everything. This would free up an extra .6 to 1 amp of 6volt heater to heat up KT88 or KT 120 that would normally push the limit of the older amps. The transformer should run a little cooler and I would think lower noise in the driver stage a little.
But of course you commit the sin of replacing the 5AR4 with diodes. That would surely send a good bottle head to purgatory for a long while. What do you think?