Often, hum is caused by a ground loop. This is a problem that is sometimes created when connecting two or more electrical components together. Sometimes the problem does not lie in any one particular unit, but in how they're connected. If your subwoofer is connected to a different wall outlet than your system, try to move the subwoofer plug to the same outlet. This may require an extension cord just for the trial. If it works, you can permanently move the sub or use an extension cord. However, if the hum you are hearing is coming from inside the sub, or if it is constant even when nothing is connected to the inputs of the sub, there is something wrong with the sub and you should bring it in for service.