The more I research about the issue, the more I think the issue is coming from a ground loop when using outputs A and B. It's a shame the relay G70 does not have a ground lift on output B. It has one in output C, but I need to use unbalanced cables in my setup.
Question for line6, can I use output C with an XLR-TS cable without damaging the receiver? Something like that, to plug the outputC to my second amp and lift the ground to address the ground loop: