Depressed by the hissing sound from M-Audio Firewire Solo

I spent almost the whole night (from 9 - 11 pm) trying to figure out why the heck there's hissing sound coming out of the M-Audio Firewire Solo when the volumn was high, and whether there's a way to get rid of it.

Result: no solution. No matter what arrangement I made, like changing cables, using a mix panel, nothing works. And the same problem happens on both the Mac and Windows side.

I guess I just have to either live with that fact, or buy a new amp that support S/PDIF cable, especially since my MacMini also support S/PDIF.