Subwoofer

My Sunfire use to do that until I had Bill F rebuild it and adjust the threshold to a lower level. Cost about $260, so you might want to consider a new subwoofer. Best wishes 

If your sub falls into standby when set to cross over at 90Hz, then either you are playing bass-shy sources or the sub is malfunctioning. Sub crossover is generally set much lower, as any quality main speakers will roll off well below that point. Play something bass-heavy and it should boom unpleasantly with sub at 90Hz. If it does, that's a good sign. So then try 60Hz, and if it still booms, go down to 50Hz.

 

+1 for what Hickamore said. 
For what it’s worth, Sunfire subs are outstanding performers and punch way above their size. Would be worth the $260 to have it repaired. Seems a small price to pay… 

Ask the manufacturer.

From my experience, subs that have auto on without a hard on/off switch are pain because thats what they do: shut of in the middle of a listening session because there is no bass for an extended period: No matter how much you fiddle with settings an auto on sub almost always turns off unless you are listening to hard rock. Most of time you would have to use a high crossover setting that might not blend well with your speakers. The few auto on subs that I have owned have been my least favorite.