Verify the polaity of those speaker cables since the sub is seeing a full range high level speaker signals.
SET UP CALIBRATION
For best results when setting up your system, assume your normal listening position and have another person perform the following adjustments:
1) Start with the Subwoofer Level Control to its zero position. Set the Subwoofer Low Pass Filter Control to 50Hz.
IMPORTENT: Set any loudness, bass/treble, and/or equalization controls on your intregrated amp to their normal or mid-point positions.
2) Play a familiar CD, Record or Video Soundtrack that has substantial bass content.
3) Gradually turn the Subwoofer Level Control clockwise until you achieve a neutral balance between the subwoofer's deep base output and your main/satellite speakers.
4) Slowly turn the Subwoofer LP Filter Control clockwise to reach the best mid base blend with your main/satellite speakers. This will be the point at which the bass retains solid impact and fullness.
If the bass becomes "boomy" or ill defined you have gone to far and should turn the control counter-clockwise to the best balance point. If the sound is too thin (ie. male vocals are not full sounding) then turn control clockwise to the best balance point.
NOTE: The Subwoofer Level Control is design to control the balance between your sub and main/satallite speakers and should not be used as a substitute for the bass or loundness controls on your intergrated amplifier. Adjust the subwoofer's level for smoothest low frequency performance. If more bass is desired, then advance the bass and/or loundness controls on your intergrated amp.
Good luck
Room placement and interaction with reinforcing and null modes will be another story.