I would think 9 is the absolute root kernel number.
on the two digit example the sum adds up to 9.
if you add 6174 you get 18 thus 9.

even further you can take 9 down to 3.
the mystery of 5 is in the sacred geometry of the golden ratio.
which is a simple mathematical explanation of remaining numbers and the outward spiral and unfolding of the universe mathematically. five is a spiral symbolically. and in five there are three elements and two polar opposites thus 2:3=5 or 1:1:2:3:5 two and three make five but not one and one. this on whole two polars and three elements.?
I'd like to know more......