The sand filling will give you a solid foundation the Fritz speakers like. We prefer a mixture of lead shot and play sand. A PIA however worth the effort. Then Blu-Tac them to top plate and be happy. As far as roller blocks, Herbies and footers in general one must know if draining, transferring, soaking up is the proper method for said piece of gear. Footers have different ways of working.
I would decouple stand from floor if a suspended wood if concrete spike it baby. If spikes are used we like decoulping speaker from stand ot Blu-Tac them. Need to try each way. Have fun as one effect may not be the same in another set up.
charles