Some people claim to use NOS power amp tubes for many, many, many years. Other people recommend changing power amp tubes every 6 months, or at least once per year. I don't think people are changing RCA Black Plate 6L6GC's every 6 months. Is the common assumption that the old stuff is of such greater quality, that you can run them for years on end without worry, while the newer stuff should be replaced much more often? I wouldn't think that the durability of the older tubes would be THAT much greater? Maybe better, but not 10-20 times greater. They didn't just start modern tube production yesterday. A few years to work out the bugs and tighten quality standards should have been plenty sufficient, even for any foreign localities.