Mark, I have your same couple, PM-KI Ruby and the SA-KI Ruby.
After your post I have checked and I confirm it is exactly the same by me.
I have the Grado GW 100, wired through a QED precision Jack-to-Jack cable.
I could hear a feeble hiss from the PM-KI headphone output, not dependant on the volume level. No audible hiss when using the headphone output of the SA-KI.
The hum disappeared when I switched off the "video" devices (TV, DVD player, Nintendo switch), unplugging them from the power outlet (I have them on a dedicated power strip with a switch). I have also unplugged the cable TV cable from its socket (even though I have put a high pass filter there, because it was causing hums with the turntable).
So, no hum, just some feeble hiss. And I noted that I like the output of the SA-KI slightly more, at least with my Grado headphones. The SA-KI is connected to the PM-KI though a 60 cm Audioquest Golden Gate, the longer signal path is of course putting the PM-KI at disadvantage, but I am not sure the difference (hiss aside) can be explained only by this fact.