Improving Power Amplifier - Digital Bias current set
T5 is effectively a current source, PNP, with base voltage (set by the diode drops of D5, T4) to roughly 1.4V below the 85V rail, giving roughly 0.7V across the 10 ohm resistor and 50R bias trim pot. If you assume say 35R as a typical setting then you get a current of about 20mA from the collector of T5. 

To shut the thing down the obvious way is to place a NPN transistor or small signal mosfet (good for more then 85V) at the bottom of R14, turning this off will remove the base drive from T5 killing the bias current. 

However, if I wanted to make the bias programmable (seems like overkill, but whatever) the way to play is to make the doings around T5 a current mirror and then build a current sink down at ground to control it, you will still need to organise temperature compensation obviously.