Tink this is just exquisite, I saw in comments that it was sung at Rachmaninoff's funeral.
I have a theory, don't know if you might be intrigued by my ideas, because I used to teach comparative spiritual traditions of the world. Those oktavists, their deep voices are reminiscent of the great OM coming out from the Source (or God if you will), to form Creation...Creation represented by the diversity of the choir voices. So humankind, we all respond at some level to the archetypal quality of oktavist tones.
Christian version of Om is AMEN, of course, anchoring our prayers back to the Source.
So I think Rachmaninoff (and the others) was actually hearing the music of the spheres, and had the talent to render that for us all to hear. Very calming...helps humankind navigate through difficult times such as right now, exquisitely hopeful.
I don't know if Elvis Presley knew about the oktavists, but it's interesting that he selected a group with one of them for his own beautiful gospel music...