Just so we are clear, to my understanding, I believe your claim is equivalent to:
Take 2 (2 * n in order to account for variations in individual instruments) instruments as close to identical as possible. Tune both instruments. Digitally record the sound profile of each one. Place one in a silent chamber. Place the other in a chamber with a music source. Ensures all other parameters of the chambers are equal (temperature, humidity, sunlight, etc.). After x amount of time, take the instruments from the chambers. Digitally record the sound profile of each one a second time. The outcome you claim is that the delta for the instrument in the music chamber is noticeably different to the delta of the instrument in the quiet chamber.
Furthermore you claim that this effect is known among guitar players.
I suggest that in addition to the effect being highly improbable, no person exists with actual knowledge of such a phenomenon, only with belief about it and anecdotal evidence that does not account for other parameters. I suggest that all anecdotal evidence can be explained by a combination of:
- survivorship bias (good sounding instruments are taken care of and survive longer)
- differences in maintenance, environmental conditions, source materials, construction technique
- differences in use (being constantly properly tensioned and calibrated compared to wood working unconstrained)
- observer bias (a beloved or famed instrument will be favoured)
I suggest that good sounding instruments are not left unplayed yet properly cared for and therefore no one has ever observed such a phenomenon. Chances are, any cared for instrument will also be played, therefore what people have observed is the difference between pampered instruments and neglected instruments.
If you actually have any evidence, please back up your claim.