Sub-woofers need a lot of power and they are not too sensitive to high frequency noise. Speaker manufacturers like to match their subwoofers up with some pretty low quality amplifiers, with switching power supplies and not much filtering.  The sub-woofers sound good, but the problem we found is that this noisy subwoofer is connected to your ultra clean, ultra precision analog output. This creates obvious noise and distortion on your main amplifier and speakers. Its like serving fine wine in dirty glasses.  It all started when we had a customer who preferred their preamp.  In our experience, there is no preamp that sounds better than having no preamp.  Noticing a powered subwoofer, we turned off the subwoofers and the sound came alive. Turned them on but disconnected the input. Still sounded great.  It was not the sound of the sub that was the problem, it was the pollution caused by it.