Believe it or not tube shelf life can be as high as 100 years...that is not a typo! I would keep them in their original boxes, in a cool place where they are not subjected to possible bumping, banging, or droping. The 'silver cap' inside the top of the tube is a substance called 'getta' which is designed to capture and hold stray atoms of Oxygen, Nitrogen, etc., which may effect the performance of the electronics. If the silver turns to powder and flakes off your tube might have sprung a leak! I my opinion tube design was discarded premature...I don't believe the engineers fully exploited possible designs.
