Look at the album (I'm using the singular because nothing was mentioned about trying another one)- is it warped, or completely flat? Even if it has a little blip, a subwoofer makes a mountain out of a mole hill and this would have been rare in the '70s or '80s because A) subwoofers in their current form didn't exist, B) most speakers didn't do 20Hz and C) most, if not all receivers had a subsonic filter that was meant to remove the frequencies that could cause this kind of problem. Look at the subwoofer, if it's direct-firing and the grill is removable- does the cone vibrate when it's not thumping?

If the turntable needs isolation, a more attractive thing to use is from Rockler Woodworking stores, called Bench Cookies. These are made for mounting or placing on a work bench while staining/finishing, using a router or doing other operations that require keeping it off of the bench top. They're cheap enough, too.

[Link: rockler.com]