I have Paradigm DSP3400 Subwoofers. Regardless of placement I cannot get response below ~24hz. After some research I have learned why. Its not placement. Its not my pre amp. Paradigm implemented subharmonic filtering on all DSP series subs. Feed them a signal below 20hz and the amp simply does not play the frequency. Martin Logan Dynamo series subs have a similar feature. This is nothing more than a governor to reduce damage from over excursion. The main reason a company would do this is to prevent warranty claims. In my case, performance has been adversely affected. Frequency limiting filters may preclude these products from being used in a system where low frequency extension is desired. This feature should be a selectable option, not a mandate. Manufacturers should indicate when such a feature has been implemented. And please, give me a button to turn it OFF.