I did hear back from Kinetic Systems and from Sounds of Silence. Tony at KS suggested I contact Steve Klein at SOS as he has more experience with the VP in audio applications. The maximum net design load of my passive 24" X 20" Vibraplane 2210 is indeed 275 lbs. Steve wrote me that there will be audible improvements when pre-loading the unit in the 125-150 lb. range and then diminishing improvements from that point on.
Neither gentleman recommended one type of slab over another but Steve did mention that a critical decision would be what material to use between the slab and the top of the VP. He recommended researching materials from EAR.
My VP arrived today and I think I will start by just setting it up as is and comparing it to my Townshend Seismic Sink. I may end up moving my TSS under my phono amp as I have another one under my pre amp and it did improve the sound. After I live with the VP under my TT for a while, I will experiment with pre-loading it. The unit weights about 150 lbs, so if I add another 150 lbs plus my 30 lb TT, that will be a lot of weight on the top of my rack.
Thanks for all the feedback and if anyone has anything else to add, I would appreciate it.