Sounds like tube rush to me.
5 responses Add your response
You might try toggling phase invert switch from your ARC to see what happen. I have been struggling the same problem from my system for a while. But, just recently I found somehow the phase invert could help reducing the noise quite a bit. Actually, I am still trying to figure out how that works. Please refer to my post with title "Hiss and hum on ARC REFII MKI" FYR.