Sounds like you had a ground loop between your M Audio device and the 1050. Why this would manifest it's self in the LFE to sub circuit is a mystery. Glad to hear you traced it.

I'm curious why it comes and goes. Is it random, or only present when the m-Audio is on or something?

I just received my SVS sub and was very dismayed at the buzz I was getting. I have a 1050 with a Pioneer DV-05 DVD player. The buzz would start as soon as a DVD or CD was loaded into the Pioneer (DTS or Pro-Logic, all produce buzz). I started randomly unplugging cables until I found that the s-video cable was the culprit. I do not have an extra s-video cable to test the cable but I found it very odd to have the buzz come from a video cable. The SVS is plugged in at the other end of the room but unsure if it is a separate circuit. I do not have video going to the 1050 any more and use the component straight to the TV. Still it is another odd cable causing the buzz.