Like I said, you're correct in that using random tubes or not matched, or incorrect biasing will result in the issues you described.
PEOPLE, DO NOT REPLACE TUBES YOURSELF UNLESS YOU'RE A TRAINED TECH! ENSURE TUBE COMPATIBILITY AGAINST THE DEFAULT TUBE SPECS THAT CAME WITH THE AMP! MATCHED ONLY! ENSURE TO BIAS CORRECTLY! lol.
My comments were basically assuming most competent tube amp owners know these basic facts haha. No matter the amp type, unless it comes with auto-biasing and can take different tube types, you always need to ensure the tube selection meets the performance requirements of the amp. I was just highlighting that if you know what you're doing, you can select alternate tubes to potentially achieve some changes in the amp's tone.