Rumble Filter for my Linn LP-12 setup ??

An isolation platform for 10 bucks .....
1 large paving slab (cement, 2 inches thick)
1 large block of foam 3+inches thick (density should be so that the concrete block compresses the foam to about 1/2 its original thickness.

Place slab onto foam and turntable onto slab. This will remove any acoustic low frequency feedback into the table. It's ugly, but it will demonstrate whether isolation is the answer. If isolation is the answer you can spend more $$$ to search for a prettier solution if you or WAF requires a prettier solution.

The woofer excursions could be caused by the cartridge compliance not being optimal for the tone arm. Are you using a Linn cartridge/arm combo? I would assume that in a Linn arm,Linn cartridges will work correctly but some other cartridges may be of to high or of too low comlpiance to not cause some osilation. My suspicion would be that for the sort of woofer movment you are describing that it may be the cartridge is too high in compliance for the given mass of the arm.
The isolation ideas are always very good even if you do have a good arm/cartridge match. I don't think you can ever have too much isolation from vibration, especially with a turntable. The stylus can read a groove deflection as samll as a millionth of an inch and any vibration at that point will also be amplified along with the signal.

Weiserb, those two folks bring up VERY valid points.

I would first work on getting the table isolated better. This can be done with sorbothane footers, sandboxes, innertubes, vibrapods, constrained layer damping, etc... The approach that you take is up to you and your budget. Combining several of the above might be necessary depending on your individual installation and situation. Keep in mind that you'll have to find a way to keep the table leveled regardless of the type of isolation that you opt for.

I just got off the phone with Kevin ( KAB ) and talked to him about the rumble filter that they sell. It is a brand new version of an older design that he's very pleased with. As it mentions on the website, this device does roll off extreme bass ( -3 dB @ 20 Hz ) and does have a relatively sharp slope ( 18 dB's per octave ). It also sums low frequencies below 140 Hz or so to a mono signal. As such, i would consider this a last resort since the effects would be audible. Having mentioned this to Kevin, he was quite confident that most people that are suffering from rumble and record warp problems would find it FAR more beneficial than a drawback. Personally, i would think that most of your situation can probably be dealt with in more effective and less deleterious ways. Sean
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My guess is that you can cure this with better equipment placement, isolation and/or room treatment. I had a similar problem after moving my system into a new dedicated room. I have since completely eliminated subsonic woofer excursions, but it took some trial and a lot of error. The combination of better isolation of the TT (a Big Rock), moving the rack to the side and away from the corner, and the addition of tube traps solved the problem, and improved the sound, too. If you can't move the rack, check out a wall shelf for the LP12, as most of the subsonics come through the floor. Quadraspire and Mana make good shelves.

Thanks for reminding me about KAB. I wanted to put in an order last night and lost my connection to the net : ) Sean
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Make sure it has a steep rolloff or it will affect the bass response. The Linn LP12 has extremely low rumble. What is causing you to get rumble? It might be better to cure the problem instead of filtering it out, which may cause another problem.