that valve is toast!!! hehe. When you look at the other valves they have some sort of substance at the bottom of the valve (shiny stuff). Isn't that put in the valve to ensure that all oxigen is "removed" from within the valve, therefore creating a vucuum tube. It just seems that that substance has decayed. I'm talking completely under correction here.
Also, aren't you supposed to remove the transporting foam from the amp? it restricts airflow

I don't know if the foam should come out. It's glued pretty seriously to the plate. And my previous set of valves lasted almost 5 years with the foam in.

the vacuum has been broken. so it is indeed toast. there is no way to prove the manufacturers are at fault, so good luck with that. part of the joy of tubes is the fact that they can be so unreliable that you want to go solidstate... but.... have you heard solidstate? haha. of course once you have got used to a brand of tube with your amp that has only failed 1 in 10 times... then its "reliable" (but you still carry a backup).