Which component is most responsible?

Of course it must be on the recording first, of current recordings the wonderful Swedish choirs on BIS lead the pack IMHO.
After that its the preamp and it not be an expensive one, the Hafler DH-101 was good at it as was Nikko back in the day.

I keep a Bryston BP-6 around for the sole reason it is stellar at this, though my EE Avant trumps it in about every other way. Of course if you use Vinyl the cart must be up to it as wel, I switched to a Nagoka MP-200 from a Grado Silver for that reason.
Any of the usual suspect 300-500S monitors will do what needs to be done.

Yes, you need a good choral recording recorded a certain way to even have a chance, so make sure that is in place first, otherwise you will spin your wheels.

Here's a good recording to try off the cuff:

Mitch Miller

Here's another:

MTC

Power cord.

(and/or wall socket).

Yes, reasonably clean power however achieved is a must for this as well.

Following up on Brownsfan's suggestion I've picked out a couple of acapella choral recordings you might like (I do anyway) which are probably still in print and might serve your purposes, although I must admit, as I suggested in my first post, that if I were trying to pin down specificity I probably would not use choral music for one simple reason. Choral music tends to be a powerful blend of voices and single voices rarely stand out especially with minimal mic'ing techniques, but even with spot mic'ing, except when used for the soloists. Anyway, here goes....

Rachmaninov's All-Night Vigil, by the Estonian Phil Chamber Choir conducted by Paul Hiller on Harmonia Mundi.

Grechaninov's Passion Week by the Pheonix Bach Choir & KansasCity Chorale conducted by Charles Bruffy on Chandos (SACD)

Sibelius Choral music by YL Male Voice Choir conducted by Matti Hyokki on BIS. This disc has both acapella as well as choral accompanied by orchestra Lahti SO conducted by Vanska. I'm partial to the acapella performances, especially of Finlandia-Hymni a transcription for all male choir of the original, the orchestrated Finlandia. I understand the transcription was to be used by soldiers matching to the war with the Soviet Union in WWII. I still get choked up when I listen to it.