tube amplifier auto bias- accurate?

I agree that auto bias is a good thing, I had very positive experience with the amps I owned.

As for tube testers, the B&K 707 or 747 are decent and can usually be had cheap at EBAY. Best of all they are transistor and need little or no calibration and won't drive you crazy with too many knobs to turn to get a reading.

Later, if you want a high end tester, the Hickok 752A is one of the best ever made but much more complicated.

This is an excerpt from Hank Wolcott in response to questions about how his amplifier works:

The circuit has to be made virtually distortionless through optimal loading before positive feedback can be allowed to increase the gain.

The output of the pentode cathode follower is direct coupled to the output stage and constant current sourced by a pair of transistors forming a current mirror.

The grid circuit of the output tubes is connected to a digitally controlled automatic bias circuit which checks and resets individual tube biases at turn-on and then is disengaged from the signal path

I don't know if that helps, I know Hank very well and could call him and ask for more details.