Woofer sucking in and out

KAB sells a subsonic filter that seems to be highly regarded. I agree with Artis4n; that's probably the only way to really solve the problem, if that's what you want to do.
-Bob

I can not recommend the KAB unit highly enough. I would not have a vinly based system (the only way I really listen to music...) without one. It's sonically transparent and, in fact, will tighten the bass up a good bit since your amp is having to expend energy modulating your woofers like you're seeing.

Filters are a last resort. All filters will effect the phasing of the signal... Find and fix the real problem.

Luna- If the degree of woofer movement varies with the record; look at the circumference of the discs, as you play them. If the excursions increase with the warp of the outer edge; you've found your problem. Barring the use of a subsonic filter; a damping ring would be the cure for that particular problem. ie: (http://www.soundfountain.com/amb/ttring.html) (http://www.ttweights.com/480_gram_alloy_outer_ring.html)

Its an isolation problem as other have mentioned - keep experimenting to find the location that causes the least disturbance. Understanding how the sound waves radiate from your speakers based on how you set them up will help alot here. Keep it away from the sound waves as best as possible.

What type of clamp or weight are you using. Before spending big dollars on rings get one of these. Used with the rubber washer under the lp it will flatten the record edge to ensure it touches the platter all around.

http://www.needledoctor.com/JA-Michell-Record-Clamp?sc=2&category=421

If you are using a weight alone in the middle it will make the lp ends go up as well.

A filter is a band-aid as other have mentioned and not a fix.

Good Luck