This voltage will depend on both the plate voltages of the 12AU7 and the health of the 6080. The difference between the 12AU7 plate voltages and the 6080 cathode voltages will be higher with the Speedball for tubes with high emission, as the tube itself will draw 30mA DC with higher bias voltage. When that voltage difference becomes really low, that's an indicator that the 6080 isn't as fresh.
Likewise a very potent 12AU7 will show plate voltages around 75V, while a really worn out one will be up at 100V (these are OA/OB on the small board up front).