If you stockpile power tubes, the warranty on them will run out before you end up installing them. Lots of places have 90 day warranties on them. I've bought matched quads years in advance and had a bad one blow on day one. No more warranty. I've only had one or two NOS or ANOS preamp tubes go bad soon after install. But since I got them from eBay sellers, who happened to be cool they took care of me.