First, make sure you pick a tub. Some of the larger tubs have restrictions on what kind of bases they can be placed on.
I own a HS grandee and the dealer really pushed either the concrete pad or a reenforced deck. Due to its weight when filled, this tub couldn't be shimmed if the base wasn't level.
But your manufacturer/warranty/dealer will probably tell you if there is a specific type of foundation that won't be covered.
I put in a patio in my backyard and can tell you that an appropriately sized concrete slab is less work (it's much easier to level concete, as it is a liquid). Assuming you have mixed concrete delivered...
Aside from that, if you go with the patio concept just make sure you tamp the heck out of the ground, the crushed stone and sand, and the concrete pavers. The danger is that over time a loose or untamped area will compress and you've got a tilting spa. A tilting spa is easy to notice because of the waterline...
Even manufacturers (like, I believe, Arctic) who don't care what type of surface you use still stipulate that it must be a stable surface.
