High pitched-whiny noise from USB audio output

After upgrading from Leopard to Snow on my Mac Pro Tower, I now hear a high pitched whine from my speakers when running any audio app, like Itunes or Peak Pro. My audio output is thru the USB, to a Digital Analog Converter (an Aeolus Audio "MagiDAC"), which is then connected to a Yamaha stereo receiver and speakers). Pressing a key on the wired keyboard activates the sound. It is low in volume. I connected another stereo and still hear this sound, so it isn't my peripherals. I've tried using the other USB connection outputs, disconnected the wireless mouse (to eliminate RF from the mouse transmitter) to no avail.

Connecting my audio out thru the old fashioned 1/8" analog plug eliminates this sound. So it is somehow related to USB. I've also moved AC plugs around, although it doesn't sound like 60 cycle hum ( too high pitched), in case it was a ground loop. I would use the 1/8" analog outs, but the USB output thru the DAC sounds better. Help, and thanks in advance!