I believe the 580V Stax bias voltage with 5M ballast resistor is safe for HE90. I have tried the HE90 with some Stax amps in the past, and I didn't notice any problem. However, if it will make you feel better, reducing the bias voltage to 500V and adding additional 5M ballast resistor are the way to go.
From the schematic, the mini T2 will be using +220V, +400V, -460V, +15V and -15V. I'm thinking of using the PS board from my prior KGSSHV build. Is it possible to use a simple voltage divider to get the +220V from +400V? How much current does each voltage draw?