I'm just throwing this out to see if anybody else is experiencing the same issues... When the Rio is powered down, there's no hum from my Rio. However, once I have it turned on and running (I'm using tRio) - I can hear a hum. Kind of like a bad grounding type of hum... (For clarification - Its quiet when powered on, just after the Rio Logo - then the tRio logo, it starts to hum...)
I really only started hearing it since I turned my speakers up and the variable line out volume control down to allow me volume control with each of the Rios I have... (Usually just have the fixed line out and ran over to the powered speakers each time to make an adjustment.)
I have three Rios and they all have this same issue... I'm going to play around with different volume settings and receiver.arf files (other client players) to see if it's just tRio, my units - or something else...