I had something short out on my 1206D, kept blowing my in-line fuse, NEVER the fuse on the amp... tried a third in-line and heard a sizzle from the amp, then went up in smoke... the 80amp fuse on the amp never blew //content.invisioncic.com/y282845/emoticons/rolleyes.gif.c1fef805e9d1464d377451cd5bc18bfb.gif.
remember, these things are budget power, not the most reliable.
what kind of sub? why did you have the amp turned on when connecting the sub? where did you buy it/how long ago?