[Bob Branstetter]

Actually, many laminated double basses (i.e. Kays) and even some carved back basses do have a disk or platform like you describe. It is there to prevent the pressure of the soundpost from deforming the back and/or prevent a crack the the back. It is possible that the disk/platform may slightly dampen vibrations, but OTOH, some have speculated that the added mass could act as a form of "graduation" on a laminated back and improve the sound. In any event, the potential sound change is very small and if it prevents a crack in the back, it's good insurance.