The bogey value for 6X4 is 24V at 70mA. Most NOS tubes seem have less than 24V, such as 20...22V, but this is no must.

The Tung-Sol 6X4WA datasheet states that specially for this tube, good quality is characterized by good performance at lowered heater voltage. (Wihout telling how low). Under this aspect, the TESLA 6Z31 is the best quality, as some NOS samples worked still as Bogey tubes, at a heater voltage somewhere 5V. Wheras NOS Tung-Sol seems to need 5.5V as a minimum.

Also the TESLA 6Z31 at the data sheet (bogey) value of 24Volt forward voltage, all draw 110mA or more, whereas NOS Tung-Sol brings 80...90mA. TESLA had their own chemical factory for cathode coating in Rosnov, so it seems they were able to use exactly the right material, appearantly better than Tung-Sol.

The Tung-Sol seem to need a short burn in. Out of the box, they bring 70mA, which is fine, but this value grows within 10 minutes to 80...90mA. Indicating these tubes are a little bit over stored, but no damage whatsoever. If they would be stored for some more decades, they would go down further, and some day comes a point where they will not fully recover any more. For that reason, a short burn in of old stock is recommended. That will bring them back to fresh condition, and they will stay like that for many decades again. General Electric tubes seem to behave similar. Only Tesla 6Z31 comes out of the 1950's boxes fresh like new made, and it seems they don't need a burn at all.

It has to be said, only the Forward Voltage (Quality test) card will make these differences visible so clear. The POWER TEST card, indicates "strong" for all of those tubes right away.