I gotta toss in my two cents on this as I personally think there is far too much digital ink spilled on this subject.
An acoustic "opening up" is not myth, but neither is it as discernible as folks claim it is.
My angle: the guitar changing its tonal profile comes with time and play, to be sure. But the bigger variable is us, our ears (and along with that our age), our perceptions, and our preconceived biases. I am a firm believer that folks all too often hear what they want to hear. That's no indictment on anyone specific, nor to the the devotees of the "Tone Rite" device, but merely a statement that
no one is immune to the bugaboo that is greatest variable of all: the human. Which is why I personally never even consider this "opening up" business in a guitar's profile,
never. So maybe the tone did get sweeter over the 9 months, 3 weeks, and the 5th day of playing. Or perhaps you just loved the tone that day. Or you are finding the sum of its qualities more appealing. Any way you look at it, it is yours to enjoy, or yours to consider selling. Was it the guitar, or is it
our perception? (Is it live or is it memorex? ...never mind

). This is not so much a rhetorical question but a literal one: which is the more
likely to have changed, my physical hearing, my perceptions/opinions, the conditions of the moment in which I am drawing that conclusion, or the guitar.
So that's me. I would
never factor such an inconclusive, inconsistent, unpredictable facet of an acoustic into the equation of a guitar's "goodness quotient." It just doesn't figure to me. Again, I am
not saying guits don't change over time; my contention is that too much is made over whatever changes
may take place. There are far more tangible, genuine facets that affect an acoustic guitar's tone that deserve more time and consideration. IMO, of course

Edward