Hi Mhraracing! sorry you haven't had any responses yet. Maybe if we bump this up you'll get some thoughts. FWIW, I'd first look at the manufacturer's site for a recommendation.
For example Hotspring says minimum acceptable is a 4" reinforced slab for their larger tubs. (potential warranty issue here?) For the smaller ones they "highly recommend" the crete route. So once you pick the tub that might have some bearing.
Beyond that, I went concrete because a slab "floats" with frost heave and stays in a relatively flat plane. Compacted base options (gravel/pavers, etc.) may work just as well. . .I know they get a lot of nods from folks on this board. But I was nervous about frost heave under one part of the installation causing the tub to "rack." Hey, I'm new to hottubs and will defer to greater wisdom. . .Whadya think guys? Give mhraracing some ideas here!
Stars