Which component is most responsible?

Assuming the recording is excellent...
1-Preamp
2-Source
3-Cables

IMO a general purpose answer to the question is neither useful nor possible. Differences between systems, rooms, recordings, and listeners will inevitably mean that the weakest link has to be identified on a case-by-case basis.

FWIW, though, if the question were re-phrased to be asking what factors generally have the greatest possibility of adversely affecting "separation of voices in a chorus or when listening to multiple background vocals," given that you are using a digital source, IMO the answers would be the recording, the speakers, and speaker/room interactions.

Regards,
-- Al

source

LOL! This thread is beginning to remind me of that old Why the rectum is the most important organ joke. ;)

Jmcgrogan2, haven't heard that one in a very long time, guess it's been around for years.

This thread is reminding me about reading this some time ago,

Put 10 audiophiles in a room to discuss a topic, and they will emerge with at least 11 different theories.