Can’t advise on that - I think you are talking about a different type of ground noise between your devices possibly related to your outlet supply and all devices being on a connected circuit to that.
In my case there is no main power, it is an ipad midi cck connected to the OP with ipad audio plugged into the OP (thus creating a loop) and there is a significant buzz : the isolator on the audio cable removes it.
However you’ve made me wonder if an isolator around the usb cable in my set-up is as beneficial as isolating on the audio cable…