Which component is most responsible?

Newbee, perhaps you could suggest a suitable recording to the OP that you know has a good separation of voices. That takes the recording variable out of the problem.

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.