How do you know when tubes are done?

I have replacement sets of Western Electric 348a tubes (my stereo amp runs four of these at a time).  But, after many years of operation, two tubes started to go weak.  Although I have replacements, these tubes are so expensive, that I have, for now, replaced them with much cheaper "equivalent" tubes.  The disparity in price is crazy--the last time I could even find a 348a offered for sale, it was being sold at $1,500 each for old, but testing "good" tubes.  The 6J7 equivalent sells for around $7.50.  Now THAT is a big difference.

Usually a browning of the getter flash indicates a weak tube, but there are a few types in which this is not the case and the tubes can still be viable. A black thick getter flash is fine. I think it was GE 6L6GCs that had a browning tendency that turned out to not indicate problems.

You sneak. Is the Yee a one-off custom or regular product? Either way kinda cool.

I have a Croft pre and power amp. I had the ffffff noise from one speaker, dropped the amps off for Glenn to service, the tube had reached the end of their lives, had them replaced. Sounds amazing now. However there was no loss of volume Glenn described it as a noisy valve!