Besides aetherical density what do you think is the difference in the four different kinds of souls? I say four as there's the presundered, the sundered Ancient, the sundered new-life, and the post sundered new life. I'm not including Dragons or other alien unsundered life as we don't know what happens to their souls. I also feel that it'd be a little silly to think there's been zero new souls made/born after the sundering. Especially if we have beings like the Pixies and Fauth who are born from souls who didn't return to the Atherial Sea.

For instance are non Ancient souls i.e the new life created by Zodiark able to awaken to the Echo even though they didn't witness the Finals Days, but witnessed the sundering?