I’m also curious how people define "neutral." With modern multitracked recording (w/ overdubs, reverb, panning, tonal choices, etc.) what is neutral? Here, I think of prepared dishes; take lasagna as an example. Is there a "neutral" lasagna? It would seem there are many recipes and many possibilities. If you serve me a hamburger, we’ll have a real problem -- but it will be a category mistake, not a problem with non-neutrality. Plus, since so much music is compressed -- mixed to be heard over non-audiophile gear -- how could a piece of audiophile gear seek to "get back" to "real music"? It's all a concoction, and one largely prepared for non-audiophiles.
With acoustic instruments, "neutral" makes more sense; there’s some degree of reality which can be appealed to; if a clarinet sounds like an alto sax, then there’s something not being reproduced faithfully. But beyond that, a clarinet can be played in different ways, in different halls, etc.
I see the word "neutral" being used to celebrate solid state amps; and for those folks, "tubey" is a put-down. (And we see that those terms "solid state" and "tubes" are largely placeholders for certain aesthetic preferences in sonic presentation.) But what’s really at issue here seems non-resolvable from the start; solid state amps (so to speak) present sound in one way, "tube amps" in a different way. Each presents something *as* music, and then each of us decides if we enjoy it. If not, get something else, roll a tube, pour a beer, etc.