Hi Marcus,
While I have not used them myself, I do know of a person who used the Grand Master series under first his Atma-Sphere MA2 Mk2.3 monoblocks, then his Joule Electra Rites of Passage OTL monoblocks. He stated the Critical Mass were the best he had ever used. I wish I could remember some of the other isolation platforms he had used, but I cannot. He spent a fair amount of money on some of these other bases, loaning one set of platforms to me (maybe, Black Matter bases (?) that were a custom size he had made for him). I would have purchased that set if the cost were not prohibitive, approx. $1800 used for the pair, they were that good. They gave a blacker, quieter background than the 3" maple blocks I am currently using. If you want, I could ask that person if he would mind if I asked him to contact you with more specific info.
Bill