[Phlogiston]

Not sure, the tube if marked AT7 should satisfy the datasheet. There are compatible tubes with lower gain as new. Would be a bad engineering practice, don't think Mac would do such thing.

Some manufactures actually ran some of the audio stages at less than 12V to keep noise down. They would also run the filaments in series and use the string to generate a bias voltage for the output tubes. I think the last one of these I saw was a Fisher.

Some of the expensive, foreign 12AX7 tubes flash like this. I have heard all kinds of explanations, none of which that I care to quote.