I have my electronic drums hooked up to a presonus firebox (via MIDI/SPDIF). I also have it set as my default sound card in windows so that I can hear my computer sounds. How can I plug my AXE-FX into it so I can hear my guitar also? I tried using an instrument cable from line out 1 L on the back of the AXE to the instrument 1 input on the front of the firebox and I get no sound...