Tung Sol KT120: Why do some of them overheat and others don't?

In this video, I want to go over what I think is likely causing the overheating problems in some of these tubes. Clearly some perform to spec, while others don't. I know at this point I am dealing with a small sample size, but I am seeing a pattern and it makes sense to me, let me know if you agree or not!
This project is using output transformers from Thermionic Labs, custom made to use with KT120 output tubes, which you can buy here:
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They also should have power transformers and chokes for this project available.
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