[simoon]

http://qualia.webs.com/

This guy seems to have done his homework, and has some pretty impressive credentials to back up his work.

There is some very interesting reading in the 'turntables' tab at the top.

He gives the characteristics on various materials used for plinth building.

[bluemike]

Check out Lenco Heaven for a few threads on Plinth building 

[neobop]

With a non-suspended table you have the option of keeping the armboard separate from the plinth.  This has obvious advantages with regard to vibration.  Cat's web site used to have vibration transmission tests of various materials.  I don't see that now.  One material that is recommended is a combination of polyester resin and bentonite. Polyester resin is the stuff auto body putty is made from. Bentonite is kitty litter - the traditional kind.  Those old Kenwood tables with the faux marble plinth were polyester and ground limestone. One thing he seems to discount is the vibration stopping ability of high mass, like slate.  What is recommended is the use of different materials like MDF and steel or aluminum.  One thing to avoid is large resonant cavities like many hollow wood tables.
http://qualia.webs.com/apps/forums/topics/show/3538558-polyester-resin-and-bentonite

With a suspended table, the arm has to bounce with the platter so a subchassis is necessary unless the whole plinth bounces.
neo