I have a NAD 3020 D (3 years old now). It started hissing recently on both L/R speakers - it’s completely silent when either the left or right is unplug, the hissing is at freq around 17.1 kHz, it can be heard several meters away after the amp has warmed up. The hissing is at the same decibel, no matter if I the master volume is at min or max. The cause is NOT due to: Speakers - I changed to different set Wall Outlet/Area - I moved to different part of the house Source - any/no source same problem. I have basically isolated it to the amp or maybe the power cable. Just wondering if I can only the amp and fix it myself out do something to remove the annoying hiss, I am currently listening one speaker because I can't stand the hiss.