You mentioned piezo's, and so I grabbed a moment to post a spec sheet. The specs for the KSN 1038, 1041 and 1056 are shown above. Pay particular attention to the frequency response curve and distortion specs - the response is quite linear and distortion at 4v is only about 1%. So even though it's a "cheapie" - its performance is good. It isn't the same as the screw-on units, but thought you might be interested in the data sheet anyway.