The changes in uniqueness/diversity that I noticed were not limited to timbre. They included nearly every aspect of the recordings. Some of those changes are, no doubt, attributable to improved RESOLUTION, but I believe that others are the result of improved NEUTRALITY.
Which still leaves me with the question about timbre. It’s something I’ve noticed before with my system, and it’s specifically mentioned as the first thing you noticed with your new, more neutral system:
This theory occurred to me one day when I changed amps and noticed that the timbres of instruments were suddenly more distinct from one another. With the old amp, all instruments seemed to have a common harmonic element (the signature of the amp?!). With the new amp, individual instrument timbres sounded more unique and the range of instrument timbres sounded more diverse. I went on to notice that whole songs (and even whole albums) sounded more unique, and that my music collection, taken as a whole, sounded more diverse.
So why isn’t there a rule:
0a) Instrument timbres sound more distinct from one another (or “unique”).
0b) The range of instrument timbres sounds more diverse.
In other words, is relative timbre distinctness a sufficient criterion for judging relative neutrality? If not, why not? Your argument, as presented above, almost makes it seem as if song/album uniqueness and collection diversity were the consequences of timbre distinctness, though I think that was not your intent. But it does lead me to wonder whether relative timbre distinctness might also be a necessary criterion.
But that brings me to:
…wouldn't criteria #1 and #2 be consequences of increased neutrality rather than standards by which we identify it?
This is a false contrast.
True, when taken out of context. But what I wrote was:
If [detecting the degree of neutrality] is reducible to timbre, then wouldn't the operationalization of neutrality be, "Instrument timbres sound more distinct?" And then wouldn't criteria #1 and #2 be consequences of increased neutrality rather than standards by which we identify it?
In which case it is the distinction between the primary observable and the byproducts of that observable. You could, for example, study the solar spectrum by observing its reflection from the moon but, when direct observation is available, simpler, and more accurate, why would you? Now, you have suggested that there are other aspects, besides timbre, that contribute to uniqueness/diversity, so there may be reasons to consider the other criteria. And you may argue above that timbre distinctness is not sufficient for detecting any degree of neutrality, so then #1 and #2 would be back to being the primary observables.
But I hope they don’t turn out to be because they are harder to apply and therefore inferior to rule #0. Generating absolute uniqueness in a complex system is easier than doing so in a simple system because it requires only a single change in a larger number of characteristics. But judging relative uniqueness becomes harder with a complex system because you must consider, and weigh, all of characteristics, many of which may be different. Judging (relative) timbre uniqueness is easier than judging (relative) song uniqueness, which is easier than judging collection (relative) diversity.
Is criterion #2 a consequence of, in whole or in part, criterion #1?
No, criterion #2 is not a “consequence” of criterion #1, because the relation between criterion #1 and criterion #2 is not CAUSAL.
I don’t understand how increasing the uniqueness of the songs in a collection would not automatically increase the diversity of that collection. That is not to say that there are not other ways of increasing collection diversity, but increased song uniqueness certainly seems like one. Can you elaborate?