I really shouldn't try to reply to posts at 4am, when I'm not thinking straight
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A hi-z input is looking for a guitar or an unbalanced microphone or some other high impedence device, not a line level source such as a sound card. Are you sure you don't have inputs maked "line in" or "line level" possibly (probably) 2 of them marked left and right?
A line level source into a hi-z input could certainly be the cause of the hum you are experiencing.
Now I can go to bed
Sam