Notions like 'good' have no measure, likewise nor does 'very good' add to that, its 'extent' is as equally indeterminate. Thus any judgment made in respect of them is relative, and as you say, we have to use ad hoc metaphysical concepts to discuss them if we're to have any meaningful discussions.

You're right, context is everything and the theoretical extrapolation of the value of something or anything in the absence of some reference - emptiness as you say - means that there's nothing with true intrinsic value. Strictly speaking, things only have perceived metaphysical values of our own making.

That said, human existence brings with it the concept of value; as humans, almost everything we think about or do is value-laden. Moreover, it does seem that the concept of 'value' or 'importance' is intrinsic to the Hunan condition, somehow it's built in. We could start with food and say our 'value' system derives out of biological necessity and then extrapolate from there. That's a long-argued philosophical discussion but it's of little point pursuing the topic here.

So what do we do when discussing music and other aesthetic subjects as here in this context? This too is complex and it's already a subbranch of philosophy but again it doesn't help much here. As I see it, in discussions such as this, trying to be be totally objective as if it were say a formal/logical argument between Russell and Whitehead serves little point. Thus, it seems to me the most logical way to discuss such matters is from a sociological perspective. So let's look at the facts.

- Mozart and some other composers from the Enlightenment and Romantic eras, Bach, Hayden, Beethoven and a few others have perennially withstood the test of time. That is, for several hundred years their works have remained consistently popular
with lay listeners, musicians and scholars alike. Moreover, there's little or no evidence that this popularity is waining in certain sectors of the population.

- Yes, classical (for want of a better word) music is in decline in percentage terms when compared to much modern/pop music. The reasons for this are complex, some are obvious, others not. What we are certain of is that the suspect metric of comparing say the popularity of 'pop' versus 'classical' music tells us nothing about the 'intrinsic' value of either (sorry to use that term again but it's in inverted commas).

- That said, then where do the key compositions from key 'classical' composers stand in what's generally accepted as the Musical Corpus?

As mentioned, we cannot put an objective measure on them but we can say that their longevity, popularity and the cultural infrastructure built up around them - from orchestras to piano manufacturers, etc. - have ensured their importance within society in similar ways to the works of Shakespeare, Goethe or Plato - his Republic etc.; that is, they are already well-known cultural icons and have been so for a very long time.

Whether any specific individual likes 'classical' music or not is irrelevant as, at present, these musical works have accumulated sufficient cultural baggage/capital to ensure their current status remains for the foreseeable future. Discussing their popularity over the very longterm serves little purpose except to say that the deemed or perceived merit of certain works by say Plato are such that they have been kept 'alive and in print' so to speak for over two thousand years, thus history demonstrates it's possible - nothing more!

- Whether any of this music is of a quality and substance that enables it to be biologically beneficial to humans remains to be demonstrated, as yet nothing's proven.

I'd only add that certain pieces
from this classical repertoire are able to generate such an emotional effect within me that they manifest in noticeable physical reactions and I know that I'm far from being alone in having such experiences when listening to music.

Why music - or why specific musical works - have the power to so affect certain people in this way and not so others isn't still clear and it's still a matter of current research.