It could also be any jack adapters used in the chain? Suspect even a poorly made terminated cable contributes to the buzz and noise.
When i used a 3.5mm male to 3.5mm female sonic gear adapter, the buzz and hiss on my ALO rxMK3-B was ridiculous.
Once removed, the hiss came down to a very slight one.
After that episode, can no longer think "an adapter is just an adapter" anymore. It does matter big time.
I tried with just the amp turned on and the 1P2 plugged in.
I'd open it up to check for problems, but I won't since it's not mine.