Volume and Connection Questions

[mindonstrike]

I really shouldn't try to reply to posts at 4am, when I'm not thinking straight .
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