Talked to the dealer. While the slope isn't likely to affect the spa (as folks noted here) he suggested I try the following if I want it to be level (which I do!
). Make a form with 2x4s slightly larger than the base, cover that with a black plastic sheet and fill with self leveling concrete. When it dries you can take down the form, wrap the plastic over the top and put the tub on top, effectively making the pad look like an extansion of the black plastic base of the tub.
Then if you ever wanted to move the tub or get rid of it (can't imagine that but you never know) or perhaps future buyers don't want it then you can just pull up the plastic and the concrete, in theory, would come with it!
Hopefully it won't break apart over time but I think I am going to give this a try!
Oh, and the tub arrives at the dealer today!
But I can't put it in until the work is done...