Some Synthesizer or hardware produce hiss when we being connected to USB especially to the high frequencies (See my article about the Connections & Background Noise).
Listen the USB Hum :
I give you a tip to remove USB Hum for under 30$ without any D.I Box : the USB Isolator.
I bought this HiFimeDIY USB Isolator for my Nord stage 2. I use USB cable to transmit the MIDI to my Macbook but some static appear in the Audio Line Output of NS2 when I plug the USB cable.
So with this USB Isolator, the Nord Stage is isolated from the ground loop caused by this connection and Audio Outputs are totally cleaned.
Another advice if you use a hub to many MIDI/USB connection between your machines. Connect the USB Isolator after the USB HUB because synthesizers can cause ground loops between them.
PS : This Tip works with Motif XF if you don’t use D.I with lift ground. But you don’t cut the ground loop, only the USB Hiss.
Hifimediy USB Isolator sold here :