Had a look at the manual for the Sunfire True Subwoofer MK IV and there is unfortunately no option to bypass the Auto on/off function. Because newer movies are usually Avengers-type they are full of low end, but if you watch dramas or listen to non-metal music, you are going to have issues. The maximum crossover on that sub is 100hz, so you are already almost maxed out.
My advice is to dump the sub and by a used Rel 5 or Rel 7.