I was going to say concrete blocks but then I found this comment on a forum:
Although this solution might be a bit drastic for "home" use, we frequently fixed the problem by isolating each turntable onto its own 4-6" metal pole, filled with cement all the way down to the building's foundation. On the top of the pole, a thick metal "plate" cut into a rectangular shape would be adhered to the pole (usually by welding), on top of which the turntable would then be placed. Keep in mind that this would be done for at least two turntables near each other, and sometimes a metal bar would be suspended between the turntable pole to allow for a professional rack-mount-styled CD deck to be mounted between the turntables. This solution never failed us, even when playing VERY bass-heavy records very loudly.
Other solutions which sometimes helped when the above solution was not available: suspending the turntables onto bed/mattress-styled box springs, which were then attached to a piece of plywood to fit under the turntable; using cat-food cans covered in rubber bands to go under each turntable "foot;" heavy/thick/dense "foam" sponges under the entire turntable; granite counter top pieces cut slightly larger than the turntable itself onto which to place the turntables; and so on.
I would like to try the cat food can rubber band method, he didn't say if the cat food still had to be in them though. Hope this helps
