The subwoofer you are using may have a power saving auto shutoff feature. Sometimes when operating at low volumes a constant bass output is not passing from the subwoofer output jack to the subwoofer, thus activating the auto shutoff feature on the subwoofer. Please try the following adjustments:
1.Set the crossover frequency in your receiver to the highest possible setting. For example, if your receiver has selections of 80HZ-100HZ-120HZ-150HZ-200HZ You should select 200HZ as your crossover frequency. (This will allow more frequencies to pass from the receiver to the subwoofer)
2.Adjust the subwoofer output level in your receiver. Raise the level until there is enough subwoofer audio to the subwoofer amp. Be careful not to overdrive the subwoofer amp when adjusting this level.
3.If listening to a Dolby Digital or DTS Digital Surround program, set the low frequency attenuator (LFE) from (-10db) to (0db). Increase the overall volume of the receiver.