If you measure after the 4.7Mohm resistor, the high value current limiting resistor in your voltmeter is in parallel, which will change the reading. This is true for any resistor, but for most small value ones the change is minimal. Measure before and the 4.7Mohm resistor is out of the loop.
Afaik, the bias voltage doesn't have to be too exacting. a 10v variance is just under 2%.
According to Kevin Gilmore, Stax phones are nigh indestructible. As I recall, he did mention one scenario where the phones could be damaged: if both amp channels are pegged at -350v, then the bias voltage could go too high and damage the phones. Pretty unlikely though.