In theory, an IC could be used to make up a DC servo for this amplifier, but in practice, it would be difficult to implement due to the high voltages involved. A better approach is to use discrete transistors. How many would be needed? If the servo feeds the top triode, just two is the answer, in the form of  a differential amplifier. The Differential amplifier strives to keep its inputs at the same voltage, which will work to keep the cathodes at the same DC voltage, when these inputs are attached to the cathodes of the top triodes. The top triode on the left finds its bias voltage from the two resistor voltage divider. The top triode on the right bias voltage is adjusted until it matches that of the left triode. Both legs of the Differential amplifier are loaded with collector resistors to keep the transistors operating within their safety limits.
  If the adjustment is to be relayed to the bottom triode, then four transistors will be needed to cascode the servo. The cascoding of the Differential amplifier is necessary because the PNP transistors that survive more than 300 volts are hard to find. So by accepting 300 volts as our limit, we need two PNP transistors in series to increase the high voltage handling ability of the servo circuit to over 600 volts. Note the absence of coupling capacitors for the top triodes  in this variation. As the only bias voltage the top triodes see is the voltage developed across the Rak resistors, the coupling capacitors may have to be reintroduced into this version of the DC servo circuit, however.