One way of looking at this is to simply go shopping for a 2005 Optima tub. Craigslist, ebay, or local dealers. If you find a nice one (you may) connect the seller with your insurance agent and see if they will buy THAT spa and have it moved to your yard.
What I'm proposing may sound wacky, but think about it: you are insured for the current value of your hot tub, and there is no better way to prove the current value of that tub than by finding a replacement tub of the same type, same age.
It also may be an eye-opener to you, as this is a great way to understand why the insurance company is willing to pay you what seems like such a low amount - but may, in fact, be totally fair.
HTH