Linear voltage controlled resistor divider
In other tube amplifier designs the standard approach is to use one or more vactrols along with an ordinary digital to analog converter. 

These devices are a led coupled with one or two light dependent resistors.

https://en.wikipedia.org/wiki/Resistive_opto-isolator

Pro: No problems with high voltage, low distortion.

Con: It can take a moment to go from low to high resistance. Light dependent resistors are very slow.