It's been suggested that the floating grounds on the outputs of sonic impact amp that I'm using might be causing the issue. Either that or (I'm guessing) a wiring error. I'm pretty sure it's not the SB Audigy 2 sound card's fault (though I've tried several others).

I've been trying to avoid this, but since the sonic impact was my only small amp, it looks like I'll have to lug out one of the solid state beasts to test out the outputs issue.

• My thread at diyaudio