I'm in FC and no problems with SciFi- which, unlike some areas, is still analog, not digital.
The notes about SFHD and 68-ant seem like messed up channel lineup (either you or Tivo), or some wierd/wrong mapping from Comcast to the cable guide.
Are you sure you set the Tivo for the correct area/lineup, when you did guided setup? there is NO HD scifi, and 68 is NOT antenna (ota). It should be analog only over cable. You might want to try/confirm guided setup and other channels.
Try tuning to 68 on both tuners, and go into system diagnostics, and see what the tuner says... should be RF analog, for one thing...