I think Stax use the new Tung Sol GTB in the 700T so that is new production. Whether you can get them anymore... I'm not sure as they are made in Russia.
As for the voltage, the GTB tops out at 450V so it's better than the regular version but still a long way from the 6S4A etc. There is also the 6BL7 but no new production that I know of.
Something else. This is a cube made for no particular reason - just thinking through an idea. This was a more hand tool intensive project, facilitated by the use of cypress which is very east to work.
The base is birch which I get locally, because birch.
The brown wood is English brown oak which is white oak from England that has been killed by a fungus that turns it brown. It's from https://www.hearnehardwoods.com/oak-english-brown-lumber-2/ I used it for my kitchen cabinets, and that bundle in the yard was one of 5 of those I bought. Pics of that later as I am still finishing the cabinet doors.
And a detail on how the leveling feet work ...