Funny thing about noise........
The first two nights that I used my FX8 with my Rivera Fandango, I did not have enough hum to bother me. It was there, slightly, but didn't change on either amp channel and it never got any louder when I turned the amp up.
Last night, I tried the FX8 with my Archon 25. I tried the Archon by itself so that I could easily identify any noise issues that I might have.
I had tons of hum. Trying to isolate it did not help much.
I ended up turning part of my cable snake into humbusters. This really helped, but there was still a little bit of hum.
I tried an Ebtech HE-2 and that took care of almost all of my hum with this amp:
Ebtech HE-2 2-Channel Stereo Hum Eliminator | Sweetwater.com
When I tried to hook up both amps, I got a crazy motorboating sound from the Archon. That's another thread entirely.........
I unplugged the Archon and left the Rivera plugged in. Now I had tons of hum with the Rivera. This was a serious WTF moment!
I started troubleshooting.
Humbuster cables lowered the hum and noise a little bit, but not enough.
The HE-2 lowered the hum but not enough.
I ended up with a combination of HE-2 and Hum-X (Fandango power cord):
Ebtech Hum X Ground Loop Hum Exterminator | Sweetwater.com
Now the amp is dead silent, and the noise floor is so low on the Fandango that I don't even need the Noise Gate block that I had been using.
Before using the Hum-X, I could hear my neighbor's power drill through my speaker.
I'm guessing that my hum on the Fandango was always there, but the Gate block was cutting it off before I was able to notice it.
Riveras are finicky amps, but man does it sound good!