Steve,
The problem is that the amp does not have a bottom and all the interior parts are in open. The slab is not only for looks but also for covering the bottom. The amp will serve the purpose of a headphone amp mostly and it is not too big, so it can be carried around. The iso-cups under the amp (between the slab and the amp) are out of question because of their size and shape. The corners must be rectangular and must have the dimensions not bigger than those of tenderfeet. I hope that clarifies things a bit. I will order tube dampers as well, but I haven't decided on the best isolation approach considering my constraints. The only way I can use the iso-cups are if I place them under the granite slab. Another question then, is it better not to use tenderfeet and covering the bottom completely with the slab and using just 3 iso-cups under the granite? Or maybe some other material between the amp and the slab and again 3 iso-cups under it?