First EML 300B failures I've heard of, and I've been observing for a long time.
@plga Lower heater voltage is NOT cause of that tube failure. I lowered the heater voltage on my Coincident Statement pre based on recommendation of owner/designer of Coincident. I had multiple failures of various 101D tubes, not a single peep of noise, or failure since install of new heater resistors. The lower heater voltage should decrease plate voltage which should give you better 300B life. I'd go back to EML with information you've gleaned from the sources you mentioned and demand they honor warranty.
These warranty issues with EML will certainly impact my choice of 300B if I ever need replacments for my amp.