Buck, floor levelor just works for a certain height. I am not sure what that is. You may want to check it out.
If the off-level is quite allot, I would just re carpenter the floor to make level. This would be done with 2x6's.
Another thing for you to think about is weight. A gallon of water weighs about 7.5 pounds, times the gallons in your spa then the weight of the occupants. My point is, is the structure going to handle this weight in one small spot?
I have a 500 gallon hot tub on my deck. I had to literally tear up my deck and beef up the joists, beams, and posts to handle this weight. After 10 years, it is sagging a small bit on one side.