I have a probem with my Waldorf plugged into the wall outlet simply because the power source is USB type B into the machine I assume. One thing I wonder is, is it confirmed the power supply itself is not the issue? Either the hub, or the hub’s power supply?
If the synth, the expesive isolators as mentioned, or I’m using a cheap DI box with a ground lift function and it works but you have to gain stage because everything gets very quiet and there’s still some noise at some frequency but it’s greatly improved. It’s just an Amazon bought Pyle brand single DI box which was around $11 on sale.
For what it’s worth, I have issues with power strips and power blocks with both plugs and usb power that when I have a few things going through them, they whine. Some are worse than others, or more power is needed to get them whining, I guess it’s the transistor? I don’t know.
A ground loop ususally sounds like DEE DEEE DEE DEE DEET, a power source whine is like EEEEEEEEEEEEEE. The power source whine is audible without headphones/speakers, the ground loop is heard in the audio chain as it is a different kind of issue, so part of determining the problem is knowing if it can be heard without speakers or headphones turned on.
I feel like this: