Im doing a production test on power amplifiers using a distortion meter
to measure THD at three frequencies % THD, % THD.
At % distortion.

Im getting a distortion failure at 10Khz which measures 1.40% THD.
No wrong value components. Because I getting a batch failure unlikely to be
faulty/tolerance in components.

What should I be looking for?

I believe this must be a subtle fault.
All I found was a PNP signal transistor was specified to be BC558B, Ive got
fitted BC558C.