sorry for not being helpful, i have an old plinius 8200mk2 intergrated and the hum you were describing is audible through my speakers as well, however im not so bothered because it's only there when it's on standby mode.
No issue while playing music.
Me too is looking to upgrade my siltech PC the back of my Plinius, Im hoping to take it to a higher level in terms of dynamics and timbre with cheaper cable, since the siltech are already very smooth and natural sounding cable, i feel that Plinius could benefit from something dynamic. Im interested in knowing what PC everyone uses for their Plinius amp. I'm thinking of dnm reson, Oyaide, lessloss, or MIT.