Are you sure the Zeners are OK, all installed in the correct direction and all the right value?Well a little bit of progress, and a backwards step..
I've replaced the Zener stack and R66, the 150k resistor at the top of the stack. However, I'm not measuring 394V on the anode of V8, I'm measuring 354V, which indicates to my simple mind that there is too many volts being dropped across R66, hence the current through R66 is on the high side. Whilst the voltage from the PSU caps is at 667V.
If it were me I'd pull the 12AX7 and see if the Zener Voltage goes to the correct reading.
You may have to measure the Voltage across each Zener. There's not a lot of space for that- be careful!