From a subsonic filter manufacturer:
I initially set my Low Frequency Filter switch to flat until I discovered some very amazing facts.There exists a phenomenon called woofer pumping aka woofer excursion. Frequencies below 20Hz are usually not able to be reproduced, and with the exception of synthesizers and pipe organs, are not a wanted part of the audio spectrum. This is especially troublesome with phono systems, since many of the vinyl discs you treasure (or wish to transcribe to CD) will be warped to some degree. Any warp in a vinyl disc will cause large outputs in the subsonic region, typically well below 20Hz. For example, a 33 1/3 RPM album with a single warped section will create a signal in the pickup at 0.55 Hz (33.3 RPM / 60 = 0.555 Hz). This is a signal that will cause significant cone movement, but is undesirable in the extreme. Not only will vented subs be completely unable to handle such a signal linearly, but sealed subs will also be stressed. Large amounts of available power will be wasted trying to reproduce a signal that was never intended to be there in the first place. To be effective, a subsonic filter has to be very steep - this allows all wanted frequencies to get through, and rejects those that will only cause problems. Note that my Citation 1 Pre-Amp's subsonic filter is set at 15Hz. http://sound.westhost.com/project99.htm