Gery

Concerning the Audio/Video switch. In my experience, when on audio, the sub is very even, fast, and clean. On video, it gets boomy. This is probably to add effects to, say, explosions in an action movie.

As far as your sub turning off, yes I have heard of this before. Sometimes, though, it is simply a weak cable connection between your amp and sub. Make sure the cable is secure and, if you can, try switching the actual cable.
On my EP125, I do find it hard to have the sub turn itself on. I sometimes have to crank the volume, or simply go to the sub and put the volume on max, then back to half.

I hope someone from Axiom can respond to this problem directly on this message board.