I haven't checked on the GENESIS, but I think the wiring is a bit different on the two machines.
Of course, both on the GENESIS and on the ARENA, the two loudspeakers are mounted in parallel (it's not stereo, there's one in the bottom cabinet and one in the backglass). For speaker #1 (backglass), the connection can be made at the terminals of speaker #2 (bottom cabinet), or at the level of the potentiometer.
The schematics are not very explicit, but on the GENESIS we find that:
2022-09-09_08h36_01 (resized).png2022-09-09_08h43_34 (resized).png
It is not said if wire 022 is connected to the midpoint of the potentiometer, or to the pin "+" of speaker #2. But the second image suggests that it is well connected at the level of the HP #2 and not the potentiometer. Which explains why on the GENESIS there is only one wire coming out (and going to HP #2).
Although it works the same, the connection diagram on the ARENA is a little different. This is what we find:
2022-09-09_08h37_35 (resized).png2022-09-09_08h42_02 (resized).png
If you look at the annotation on the second image, it is said "AUDIO FROM SPEAKER #2". Sure it's unclear where it is connected !
But as the schematic has been modified, and the ARENA produced later than the GENESIS, it's probably because they bring a change to the connection. If you look also at the second pin of speaker #1, it is directly connected to the "ground".
This suggests that HP #1 is no longer mounted directly in parallel to the HP #2 terminals, but is connected between ground and the midpoint of the potentiometer. Electrically, it's the same, but it's done differently.
So, I think that on the ARENA, the wire 022 is connected to the middle point of the potentiometer, while on the GENESIS, it is connected to the pin "+" of the speaker #2.