The JBL Sub 135 is a powered subwoofer for home theatre. The unit produces low-frequency bass signals when connected to an audio-video receiver with a playback source, such as a DVD or Blu-ray player encoded with multichannel audio. If your JBL Sub 135 produces a thumping sound, try adjusting the control settings and making other adjustments before disconnecting the component and taking it to a shop for repairs.
Pull the JBL Sub 135 away from the wall, which can absorb and reflect vibrations from the sub.
Dial the gain and volume controls on the back of the subwoofer counterclockwise while playing an audio source until the thumping stops. The gain (power) control may be set too high for the sub to handle the signal from the AV receiver.
Lower the volume setting on the AV receiver to reduce the signal output to the JBL sub.
Adjust the JBL's crossover control on the back panel counter-clockwise to direct less of the AV receiver's signal output to the subwoofer. Higher frequencies will be sent to other speakers connected to the system, reducing the overall load on the JBL.