I've actually corresponded with Nagyvary on this very topic. He was not a "chatty" guy, but he responded to several emails.
I've read a lot of what he's published. I suspect that the strad instruments (and also the other cremonese luthiers who were Stradivari's peers, as these are noted for being extremely toneful instruments as well) benefited from a unique fluke of a series of events that have never before and will never again occur. His artistry as a luthier should be considered first and foremost, but to say that no other luthier that has come after him could have duplicated or surpassed his skill is ludicrous. What about the crop of Cremonese luthiers taught by Stradivari, Amatti, etc. etc? Shouldn't their violins have similar or better properties?
No, there was something else at work. Several theories abound:
* Wood stewed in various substances (urine is one mentioned substance along with some kind of fruit, can't remember)
* wood that was shipped via river (floated down the river) from the alps and soaked in lakes/canals for years before it was used may have leached out sap and impurities and left the pores larger and hollower
* The ground that the cremonese luthiers used may have had "salt of gems" (powdered semi-precious and precious gems) put in it, it may also have been partially made of glair, potassium silicate (sp?), and may have included boric acid (actually, a few modern luthiers swear by several of these theories) - the ground is important, because it is the only part of the original finish that is still present in any large amount. (think of it as a modern day pore filler, etc)
Actually, Nagyvary has supposedly recreated Stradivari's ground and also uses wood recovered from the great lakes that has been underwater for at least 75 years. It is very old growth wood.
I'm sure its a combination of several factors at play. This stuff is fun to experiment with. I've read just about everything there is to read about it on the internet. I've actually made my own glair (whipped egg whites, gum arabic, and honey or sugar) but added bentonite clay and tea (for coloring) and used this as a ground on a maple strat neck. The final finish was 8 coats of pure, raw Tung oil (Behlen's). It looks very nice and is highly resonant. It cured in the sun for over a week.
Obviously not a time tested finish for electric guitars. I'm not advocating it, just sharing the result of an experiment. I know that I will be ridiculed by the majority for being "wierd" but it floats my boat and helps me find my lost remote, so everything's going to be all right.
Yeah, yeah, this is up there with food coloring as a dye, lets not go there, ok?
To add to the "Cliff Clavin-ness" of this quote, did you know that Stradivari made more than just violins? He also made Violas, Cellos, Basses, Guitars, etc. etc.