Normals don't understand the difference an adjustment can make. You can tap the sound post half a millimeter and feel like you're playing on a completely different instrument.

I'd say they proved conclusively a modern violin you can adjust is preferred over an old violin you can't adjust, and pragmatically that's the only meaningful test since since there doesn't seem to be such a thing as an older violin you are allowed to adjust freely.

I believe rare instruments like those are always kept in pristine conditions, and nowhere it is mentioned that the new ones had adjustments done, you can assume they all were received in 'normal playing condition' from their owners.

You believe wrong. Also, even if they are, traveling with an instrument, especially and old finicky one (and they do get finicky when they get old) can throw it out of adjustment, a seam can pop open, a string can go false, etc.

The new ones, on the other hand, since they were actually new, were probably being maintained by their makers, who undoubtedly made sure they were in top condition when they went into competition with a strad.

It's a reasonable assumption to make that the new ones had adjustments done. Even larger was the unchanged strings. I could easily imagine that the older ones, since they're not allowed to be modified in any way, had older strings than the new ones. That alone could produce a huge difference.

That's a difference, but it's a difference with obvious large implications for using the things in performance as well as experiments. As such, it's hard to see how it's a problem in the experimental design; if you're not allowed to adjust the Stradivarius while it's in your possession, maybe a newer model really is the right choice for a concert?

That is fair; but, my assumption in this situation is that if the musician owned the instrument themselves, they would have been allowed to tune it. Not being allowed to change details may have been a way for the benefactors of the instruments to keep them from being harmed

With Stradivarius violins costing millions and millions of dollars, I always assumed that the people playing on them didn't, in the typical case, actually own them (much like how science is now mostly practiced by professionals funded by grants, instead of by the bored idle rich). But I'm not informed on that point; it could be very different.

I don't know the exact breakdown of numbers, but it's increasingly common for a Strad to be owned by a benefactor and loaned to the musician. I don't know of any other artifact that straddles such extremes of being a treasure but also a work horse. Even the owners of lesser instruments have to deal with the implications of taking a valuable and delicate instrument out of the house to play in a saloon.

For this reason, tweaking a Strad is unlikely to be done casually. Re-stringing? Sure, at the very least E strings, which break all the time. But a set of fiddle strings is damned expensive, and takes time to break in so they don't sound too harsh, so re-stringing has to be coordinated with one's performance schedule.

Online, one can find a list of every known Strad, its history of owners, etc. Interestingly enough, a couple of them are listed as missing or stolen.