What is the size and filled weight of the tub?
Larger tubs (5000-6000 lbs filled w/6 people) we suggest a 10X10 4" concrete reinforced slab.
If you are talking about a smaller tub, pea gravel should be fine. I have had a customer install a Caldera Martinique on pea gravel with no problems.
Personally, I would go with a slab less likely to go off level.
Good Soaking,
Neil O'Donovan
Phoenix Park Outdoor Living
Wilmington, NC