The known technical aspects of "running in" (we call it burning in - but then, we cal it "English" too!) have to do with the cathode.
The cathode is fairly complex and dynamic at a molecular level, and was only beginning to be understood when transistors supplanted tubes and research lost interest. Two things are known though:
1) the cathode must be formed. This is done in the factory, and consists of running some current through it; sometimes at slightly elevated temperature. It normally takes some 50-100 hours. Supposedly it brings or exposes individual barium and strontium atoms at the surface. Also, some trace elements of the substrate alloy diffuse into the coating. (In fact, a bit of silicon in the nickel substrate is known to speed the forming but shorten the working lfe of the cathode.)
2) many kinds of cathode will form a resistive layer between the substrate and the coating if they are operated for a long time with no current. This layer limits the emission, and adds noise. It is plausible that it can be reversed by running some current.
I am not aware of any studies of long-term storage, but since cathodes are known to be chemically active through their life it seems reasonable that the would require re-forming if they have sat idle for decades.
For what it's worth, I measured some of the earliest modern 300Bs (I think they were Sovteks, the first production run) a long time ago and found that their cathodes were not initially capable of providing their proper current. After 24 hours they were significantly improved, and after several days they met their specs easily, and I ran one pair of them for several years with no problem.
Also for what it's worth, remember that the getter, which traps ions to maintain the vacuum, only works when it's hot. A tube that has been idle for an extended period may have become slightly gassy, and operating at temperature would allow the getter to resolve the problem.
