My MarkI has these tubes, but with a #6 hardness rating. I did have one of them go microphonic. You could hear a rattle after a palm-muted barre chord, which was F or F# if I remember correctly. It was my first microphonic power tube that I can recall. The rattle sounded very much like the tube rattle that is sometimes heard out the rear of a cranked combo, only this came through the speakers.
That was precisely why I abandoned combos years ago. I have returned to a combo setup for my Mark IV though but as soon as I get tube rattle back to the head cab it goes or maybe even into the rack box. That tube rattle can be so annoying...
