WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I have once studied this subject.
I summary, quite different kind of tubes can be used in PP-amplifier with very low distorton values. This requires - in case of fixed bias - that the bias of both output tubes can be adjusted separately to same cathode current and the output of phase splitter AC-voltages can also be adjusted to minimum output distortion.
I suspect that if any global feedback was disconnected, then measurement of output distortion as signal rises would be a good determinant of output stage valve matching, as it measures a consequence of imbalance across the whole dynamic range - both from the valves and their imbalance effect on the OT.
It would be interesting to know if anyone has checked this for a reasonable number of matched/unmatched pairs to appreciate the spread in distortion levels experienced.
Another measurement for fixed bias PP could be to meassure the voltage signal across individual cathode sense resistors, and do a precision rectification of each signal, and compare the DC levels through each PP valve. For large signal levels, this may be a good indicator of the DC biasing actually being experienced by the OT (compared with just matching at idle level).
A current transducer that can meassure DC at low levels may also be able to measure such imbalance - eg. by passing both anode leads antiphase through the transducer (eg. a LEM, or tek current probe).
I guess another way could be to apply a dc current to an auxiliary winding on the OT (eg. use a separate secondary if available, or wind a separate turn or two if accessible), and then check if output distortion is minimised with bias.