try wall-mounting on something like a target wall-rack for turntables, or build yer own...
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Just some sympathy here. Whenever I have the grills off and play an LP I can see the woofers pumping or oscillating out of time with the signal. It is scary looking, and varies in proportion to level, but I can't hear any trouble. It occurs regardless of the speakers or amps used. Just on LP though. Thanks for the great post! Maybe you'll get to the bottom of this phenomenom.
What is being described is a classic low frequency resonance problem. In as much as you are using a table and substructure that are both suspended (and more importantly, un-damped / under damped) you are likely experiencing the effects of a high Q system. With un-damped systems, you can experience mechanical resonance with amplification effects (Q greater than 1). Reconsider your approach, use only one suspension component that is properly damped (Q = .5 to .7). Kevin Halverson
I think you could be seeing your (I assume) less than 10" woofers trying to reproduce 15 to 30 Hz information on your records which your speakers may not have been designed for. You should not have this problem with your cd's, as they do not produce sub 20 Hz information. My first guess is to agree with Grumpybb that you need a subsonic filter to control this, but not all preamps today have this. However, there are occasions where better isolation will improve this if it is a speaker/turntable interaction, as your cartridge is a contact microphone and will reproduce any vibration that touches it, including bass your speakers produce that gets through your Linn turntables isolation. the Linn is a well respected turntable in some circles (we all have our preferences) but it is not the last word in isolation. Therefore I would put your turntable on your seismic sink and on a Target wall mount stand (made for turntables in England where they own a ton of Linns) which will yield improved resolution by itself, and also spike your speakers if they are floor standing. I had to make assumptions since you did not list your speakers, if you could tell us what speakers you have it may help.