The rights and tooling don't get ya the most important part, the people who handle the pieces and build the tubes. I'll go out on a limb and say the last "Real" WE300Bs, came out of the Bendix plant in KC MO. They moved the tube operation to ARK, but I never heard of them actually getting anything built? Really good tubes almost need generations of makers that pass down the art of tube making IMHO.
Remember that brand new outfit that was going to start making ultra high end 12AX7s a few years ago? There was a big thread on that deal, but I never saw a single tube for sale, always prototypes. I just don't think it's so easy, and look how the Chinese and Russian tubes are getting so much better now. I think it's plain old "know your trade" type experience.