Subwoofer

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. 

Rel makes fabulous subs.  I have a pair of S/5 SHOs and in addition to adding weight and expanding the sound field, they absolutely disappear.  Subs should crossover in the 30 to 40 hz range.  They are subs, not woofers.  Much higher than that and they won't disappear and will give you a boomy sloppy bottom end.