Please read the last paragraph of this 1991 paper:
http://www.kongsfjord.no/dl/Antennas...ensitivity.pdf

Generally, a vertical antenna is ideal for DXing since its main "lobe"
is
towards the horizon (in all directions). But I know you knew that.
Could
that account for the difference in S/MMN ratio in the Kongsfjord
comparison?
....
Additionally, in the Kongsfjord power line comparison (45Ft vertical Vs.
Loop) the vertical's main lobe is at the horizon (under the powerline).
The lobes of the loop are very much like that of a dipole (above the
horizon) - more closely aimed at the powerline.

Addition part II:
The powerline noise is horizontally polarized while a vertical antenna is
vertically polarized. The noise was 90 degrees out of phase [typically a
difference of what? 6db?]. Thus the vertical antenna in this situation was
at a distinct advantage.
Out of phase and major lobe pointed away from the powerline.

A square loop, as is the ALA100 loop, is both horizontally and vertically
polarized. Thus more sensitive to receiving the horizontally polarized
powerline noise compared to a vertical antenna.