Dertonarm is exactly right. Same advice applies to the Minus K platforms - you need to have the total load weight (table + additional slab) come as close as possible to the maximum rating for that platform in order for it to work best. This is necessary to fully 'load' the springs.
Vibraplane: Should it be loaded close to limit?
I just bought a Vibraplane 2210. It has a maximum load of 275 lbs. I plan to put my unsuspended SME Model 10 turntable on it, replacing my Townshend Seismic Sink. My question is this: Because my turntable only weights about 30 lbs, should I preload the Vibraplane to get the total load with my turntable closer to the maximum load? Will this improve the isolation? If so, what do you recommend I use to preload it? I was thinking a sheet of steel, aluminum or granite. Any advice would be greatly appreciated.