So this issue is not as Scary as it seemed to be. Pun intended.
Ah, JJ EL34 power tubes, Sort of reminds me when I tried a quad of their 6CA7 tubes. I had one tube that kept cutting in and out. It was in a Mark V90. The tubes sounded great but that only lasted 2 weeks. Had a different sound, more like the amp was red plating.
If the tube is cutting out, you can see that if you can view the cathode, there should be a blue corona around the cathode if the tube is operating, if it cuts out that blue corona goes away. I found that most EL34 tubes do not have that blue glow on the glass like most 6L6 tubes, but if there are holes in the plates, you can see the cathode. Along its edge will be the blue color, if you look directly at it, it will appear orange.