Trying to understand you here, Ed. I very much respect and appreciate your many contributions to the DB Forums and have learned much from them.
Well, no. WHAT tone one wants can be subjective (I want a darker, less nasal sound), but the ASSESSMENt of that "tone" isn't (this bass sounds thin and nasal).
Perhaps I should have amended my previous statement to say "the
interpretation of tone is subjective." I think we agree here. However, how is "thin and nasal", by its nature, an objective measurement or description? It sounds like an opinion.
Music by its nature is aesthetic, and therefore subject to the preference of each individual listener. If you say a bass sounds thin and nasal, and I say it doesn't, how can you tell me that I'm wrong? (Maybe that's the first bass I've ever listened to and I have no frame of reference. And I liked that sound! Gee, thanks Ed.

) Getting out a frequency analyzer and comparing and contrasting the spectrum of a 'warm, dark' bass to a 'thin, nasal' bass - now we're getting somewhere. Now I have at least a better idea of why you're saying that.
What do you mean "proven"? If I hear, for example, Peter Brendler playing his bass and I think to myself "Man, that bass has a nice strong fundamental and big, warm sound." and then I talk to my buddy Jim Hershman and ask " Hey have you heard Peter Brendler play?" and Jim says "Yeah, he gets such big, warm sound." and then I talk with Nathan Peck and ask him the same thing and he says" Yeah, what a great sound, it speaks so well and has (wait for it) such a big, warm sound." then I think that the consensus is pretty well supported, I don't think that any of us mean SMALL when we say BIG, or COLD or BRITTLE when we say WARM, or that the immediate voice of the note is obscured or lost the we say PRESENT or SPEAKS WELL or anything that implies the presence of a strong fundamental when the string is addressed.
Objective (adj.) - (of a person or their judgment) not influenced by personal feelings or opinions in considering and representing facts.
These are "in the ballpark" descriptors of tone accompanied by the relative consensus of others when analyzing and discussing it. For all practical intents and purposes it's good enough language for many players to be able to arrive at a sound they like and that and works for them. I do get that and the usefulness of it. However, nothing in the above quote presents any truly objective information. It's all just a bunch of opinions.
There are, of course, many extreme, and perhaps some not so extreme, examples on which almost everyone would agree on. That's not enough to argue that listening can be an objective experience. No matter how close it may approach objectivity in some extreme cases, it never fully gets there, in my opinion.
Looking at your thread title, I wouldn't say "improving" so much. Age and vibration aren't going to make a bad sounding bass into a good sounding bass. But the sound does become a richer and more nuanced version of that original voice over time and playing. So yes, if the bass has a good sound at the outset, that sound develops in a way that becomes objectively "better".
To try and (respectfully) drive my point home here, I don't precisely know what you mean by "richer and more nuanced version of that original voice". But I can guess - Is it a result of the bass "opening up"? (I suppose that would be the bass improving) Or through really knowing how your instrument responds to you and getting the most out of that? (That's not the bass - that's you improving!) Or both? Or some other factor(s)?