Why are you so hung up on using the USB input?
Especially considering that the USB gives you hum, while the optical input resolves the problem???
Use the optical input and be done with this non-sense!
EDIT-- "High Pitched Hum" does NOT suggest "Ground Loop Hum"........Ground Loop Hum is a distinct 60Hz, the same frequency as the mains power.
I will tell you from personal experience, USB connections suck! I have had many problems due to USB.....granted that most of my problems have been in industrial applications. But, my experience and headache with USB has absolutely driven me to choose USB when it is the ONLY option available.
USB has 2 advantages:
1) It is ubiquitous and easy to plug in
2) It carries power on the same connector as the signal
One question for you, How long is your USB cable?
From Wiki:
The USB 1.1 standard specifies that a standard cable can have a maximum length of 5 meters (16 ft 5 in) with devices operating at full speed (12 Mbit/s), and a maximum length of 3 meters (9 ft 10 in) with devices operating at low speed (1.5 Mbit/s).[77][78][79]
USB 2.0 provides for a maximum cable length of 5 meters (16 ft 5 in) for devices running at high speed (480 Mbit/s).[79]
The USB 3.0 standard does not directly specify a maximum cable length, requiring only that all cables meet an electrical specification: for copper cabling with AWG 26 wires the maximum practical length is 3 meters (9 ft 10 in).[80]