1. Try a 'Y' rca cable connect the receiver LFE to both L/R line input on the sub, if made no difference then
2. If your AV Receiver has pre-out and you have a pair full range front main speaker, you can try connect the sub line input to the receiver front L/R pre-out, and re-configure the receiver setup to full range main L/R speakers, select ’No sub’ for the LFE channel, set the subwoofer internal low pass filter to 80Hz or lower. Hopefully the stereo full range pre-out signal will wake up the sub easier.
2. If your AV Receiver has pre-out and you have a pair full range front main speaker, you can try connect the sub line input to the receiver front L/R pre-out, and re-configure the receiver setup to full range main L/R speakers, select ’No sub’ for the LFE channel, set the subwoofer internal low pass filter to 80Hz or lower. Hopefully the stereo full range pre-out signal will wake up the sub easier.