When I said there are sounds the musician hears and feels the audience cannot, I wasn't referring to the ability to pick out different sounds, but rather the physical impossibility of certain sounds (e.g., higher-frequency overtones, squeaking strings, rasping strings) to be heard beyond a certain distance away from the instrument being played.

To go off on even more of a tangent, I sometimes feel bad that not enough people get a chance to listen to musical performances up close. I mean really close, with the musician in front of them in the same room so they can hear "almost" everything.

"Almost" because, at least for violins, you can't hear or feel everything until you actually play the instrument (can any violinist here vouch for this?). This is especially so because the chinrest transfers some of the vibrations to the ear through the bone, and there are sounds made by the bow and strings that only the person playing can hear simply because they are too quiet, and that only during quiet pieces, etc.

I've never played a Strad, nor do I have much experience, but I wonder if nearly irrelevant details such as these are a part of what sets the best instruments apart from the rest?