Which component is most responsible?

Nothing is possible unless your speakers can accurately reproduce the signal it is fed. If they can do that then the rest is found in set up and electronics. Not necessarily easy, but possible. Order of priority of electronics Amp to match speakers, source for producing a signal and lastly a pre-amp. Lots of folks like the last first, likely because its effect is more subtle than the others. Personally I think it is easier to fit a pre-amp into and existing system than the others unless you are buying/building a really high end system and the subtle stuff is going to be critical.

FWIW, separation of voices in a chorus is very difficult to achieve UNLESS it is in the recording in the first place! And it usually isn't, at least in my experience. Be sure you have a recording that really has the distinct separation of voices you are seeking. The clarity that you seek would also be apparent in the 'soundstage' presented by an abundance of recordings and would not only be apparent in the separation of instruments. That is just a starting point. Once again it is clarity of individual tone as well as location of the instruments.

I can not comment on the performance of your Polks. I've never heard them, but that said before I decided to ditch them I would be sure I had them, and my room, optimally set up so I could tell if they really were deficient or if it was just premature up-gradeitis.

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.