Subwoofer

most subs havean auto sense switch you can turn tat switch offand set the sub to on

My SVS goes to sleep shortly after the music stops, but you can set it to stay on all in the time in the software, if you want.

Oh and you might want to turn the top end of your sub down to maybe 10-15 hz above where your main speakers start to bottom. Example, my speakers go down into the 35 hz range and my sub rolls in at 50hz, so there is only a small overlap. If done right, you shouldn’t be able to hear where the sub is.

All the best.

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…