My guess: It's jitter on the USB input. The measurements in the recent Stereophile review of the DacMagic confirm this:
http://www.stereophile.com/digitalprocessors/cambridge_audio_azur_dacmagic_da_converter/index2.html
Their conclusion: "...its USB input is really of only utility quality and shouldn't be used for serious listening."
Jitter results in frequency modulation that can present itself as shrillness in the treble. I can tell you that, when I addressed jitter in my system (with a reclocker), my high frequency nasties improved dramatically.
Good luck!
http://www.stereophile.com/digitalprocessors/cambridge_audio_azur_dacmagic_da_converter/index2.html
Their conclusion: "...its USB input is really of only utility quality and shouldn't be used for serious listening."
Jitter results in frequency modulation that can present itself as shrillness in the treble. I can tell you that, when I addressed jitter in my system (with a reclocker), my high frequency nasties improved dramatically.
Good luck!