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If the earth where the spa is going to be placed hasn't been tampered with for a number of years, you should be ok without the 2nd layer of base. Now would it make it easier to level the pavers??...yes, it most likely would. Unless you have an Artic tub with the Forever Floor, you will have to most likely install a foundation that is a minimum of 4" thick for your spa to rest on for warranty purposes.Since you're already installing the pavers around the tub, why not just cover the whole area and place the tub on top. This will give you a much stronger foundation and keep you from voiding your warranty.Btw, if you do decide to cover the entire area with pavers, I'd suggest using crushed limestone for the base... installing it in 1''- 2'' layers, compacting it after each one. Then lay down atleast 1'' of sand, set your pavers, and then go over the entire finished-set patio with the vibrating tamping plate.
From a tech with a previous life in highway construction/sand & gravel:1. You should be fine without the stone dust (as long as your soil doesn't have too high of a clay content - i.e. it should be sandy).2. I always recommend a minimum of 4 inches of gravel to start. Although it won't void every warranty if it's less - that's manufacturer specific. Call Saratoga if you need to.3. Try not to use pea gravel - use a more appropriate material. I build with 3/4" crushed rock. Problem with pea gravel: it's too round - therefore does not compact the way you want it to. Crushed rock (1/2" or 3/4") is my recommendation. This is because it is more angular, and will allow induvidual pieces of material to "lock together" - as opposed to round pea gravel, which will slip.