Well, I just received my isolation transformer. I put it in the chain like you said but it didn’t work : “This specific frequency is indicative of a ground loop in the system. You can try placing an isolation transformer in between the amp’s FX send and the H90 and the H90 Out to the effects loop return. It’s possible only one of these will solve the issue.”
Luckily it worked when I put it before the amp input and with a groundlift.
Does this make sense to you, and is it safe?