Interesting. I notice if I disconnect the phono cables from the power amp input (I use this as the to power the front channels from my avr) the hum is reduced. TBH, I've stopped worrying about it as it's not noticeable in day to day use. This is my first experience of high end hi fi so I thought issues like this wouldn't occur. Live and learn.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.