Which component is most responsible?

the preamp is the key to the sound you are describing.

Happy Listening.

Awhile back I started a similar thread. For me, the holy grail was a clear spatial separation of the first and second violins in string quartet recordings. My premise was that the problem was inherent to my speakers (magnepan 3.7Rs) due to the side by side arrangement of the ribbon tweeter and the quasi-ribbon midrange.
I got a variety of suggestions from power treatment to other speakers, even headphones. In the end, the solution was realized by a preamp upgrade. You may find that you will find the biggest improvement by replacing the weakest link. In the end, every component in the chain has to be up to the task. Speakers can't recreate a spacial relationship that your CD player smeared, but the reverse argument is equally true.

This has to come from the source or else nothing downstream can put back what was lost. Massed chorus music and complex full orchestra passages are extremely challenging to the entire audio chain so that it all needs to be right or else muddiness results... much easier to get distinctness with a few "voice" lines than many competing voice lines

Which component(s) is most responsible

Components is correct, all of them. As Davide256 said, "This has to come from the source", then the preamp/amp must do its job to drive the speaker with this quality created by the source, and the speaker must be able to reproduce this quality. Just having all high quality components/speakers will not assure that you will achieve this quality. Synergy between components/speakers is extremely important as well.

I disagree about the 'source' theory. I have heand the clarity of separate voices in a choir, or individual violins in various systems. and most of them had 'ordinary' sources. Cheap Cd players.. Generally the ones which can 'do' the job have a good preamp, a stellar amp and great speakers.
I heard this phenomenon first time back in 1984 when I listened to Carmina Burana auditioning speakers in Chicago with Infinity RSIIa speakers (EMIM units in there) Powered by a Onkyo Integra amp, the big one. And a Integra preamp. Ordinary second generation $400 CD player. No special wires or anything..

So I challenge the 'source' folks to explain it...
Maybe it was a 'magic' CD player?

Now it is true a great source can sound marvelous.. But many cheap ones can sound really fine with great maps and speakers. (Maybe they just never heard a good inexpensive source?)