I have a DirecTV C51-100 box connected to my Slingbox M1 via (A/V Out to Composite RCA) cable. Then I've got RCA Composite going from Slingbox to DVD Player. The DVD player is then connected to the TV. For some reason with this setup I can only watch the Slingbox when the TV is turned on. If anybody can help, I'd really appreciate it.
I would check to see if your cable box powers off when your TV is powered off, or if the cable box powers on when the TV is powered on. If the cable box is powered off, the Slingbox won't get any signal. Here is an article that discusses what I think is going on:
Hope this helps!