For instance, the one you can get at Radio Shack specs a frequency range which effectively removes the lowest frequencies from the audio signal.

Dang. Used those before. Off I go to Ratshacks site to check it out. Hmmm... Something odd here...
Under "Specifications" it lists some alarming numbers for the frequency response. That can't be right, everything would sound like it's being played through mud. But under "Diagram", the freq response curve is laudably flat:

>http://support.radioshack.com/support_auto/9643.htm

Any idea what's what?