Woofer motion problem.

Viridian...A properly designed rumble filter eliminates difference signal between channels, which is vertical groove modulation, and which constitutes most of the problem. Some overall attenuation below 20 Hz, such as provided by some preamps, is also desirable, but getting rid of the difference signal is most important.

By the way, when LPs are mastered, for various reasons, low frequency differential signals are cut, so you don't loose any real music signal when you use a rumble filter.

El, which is exactly why I agreed with you in my post. However don't discount that there is a consequence to every solution and the in-band phase shift of the rumble filter is one consequence as well as adding an extra pair of interconnects and the associated high-pass components, which is why I suggested a phono stage with a built in LF rolloff or switchable HP filter as an alternative as it would at least elliminate one more box and the associated interconnects.

I have also heard of people having this problem when their table/platter is not perfectly level. Upon leveling their problem went away without resorting to the use of a filter.