I don't know why or where this "myth" originated, but there are instances where, at least from my experience, the military version of a tube far outclassed the civilian variant: the Sylvania 6SN7 JAN CHS VT-231 and the RCA JAN 2A3 grey plates.
From what I understand, JAN tubes are manufactured to a military specification - not necessarily for the military. One of the most important parameters these tubes must meet is resistance to failure in high vibration environments and manufacture to tighter tolerances. For an audio application, this could mean a less microphonic tube. Not only that, but the mil-spec also dictated how they're packaged, shipped and stored - which means that the tubes found today probably had a better chance of surviving long term storage. Are they all better? Can't really make that blanket statement - because many of the commercial RCA/Sylvania/Ken-Rad/Philco/National tubes already met Mil-specs. But some specific variants ARE a different tube.
From what I understand, JAN tubes are manufactured to a military specification - not necessarily for the military. One of the most important parameters these tubes must meet is resistance to failure in high vibration environments and manufacture to tighter tolerances. For an audio application, this could mean a less microphonic tube. Not only that, but the mil-spec also dictated how they're packaged, shipped and stored - which means that the tubes found today probably had a better chance of surviving long term storage. Are they all better? Can't really make that blanket statement - because many of the commercial RCA/Sylvania/Ken-Rad/Philco/National tubes already met Mil-specs. But some specific variants ARE a different tube.