Spider V20 - oscillating hum when 2 are connected to a Yamaha MG82cx mixer.

[storyteller164]

In a desire to upgrade the amplifiers we have for the sound system (folk musicians) We got 2 Spider V20 (NOT Mark II) amplifiers. 

The mixer is a Yamaha MG82cx - 8 channel, phantom power. etc. 

When the amps are both connected to the mixer, when powered on - the amps have a high-pitched oscillating noise that sounds like something from a 1950's sci-fi movie. 
Troubleshooting so far: (None eliminates the sound) 
* Swapped amps and outputs

* Set the selector to acoustic /clean and turned off the effects settings. 
* Tried different and brand-new cables. 
* powered on one amp at a time
* Tried with the mixer board powered off. 
* Used a splitter to connect both amps to one output channel. 

* Connected mixer and amps direct to wall power and different power strips

No oscillating sound when: 
* One Spider V20 and a different amp. 
* 2 non V20 amps

* Only 1 V20 connected to either output

 

Suffice it to say - I'm having a hard time figuring out this one. 
Any suggestions are welcome.