First, where is the hot tube? Is it just on the deck attached to the house or is it separated from the house? How easy would it be to get wires to it?
Sounds like you just want a pair of speakers by the hot tube with Jremote to control the source. That is pretty straightforward. The most reliable way to do that, imho, is the old fashion way - run wires from zone 2 of your receiver or from a separate amp. There are plenty of outdoor speakers that will work in that setup and you can just use your iPad as a remote to your existing system.
If that is out because you do not want to run wires, then you need a wireless speaker setup, which also means you probably want AC power available to drive the speakers. Battery powered speakers in the snow in winter is probably not a good idea. Your choices of wireless speakers is also more limited.
If you can narrow it down to wired or wireless that will help the decision.
A wired solution will let you easily change speakers over time. The wireless system typically requires you to replace everything if you want to change the system.
I have had an inexpensive pair of outdoor speakers (Cambridge Soundworks) by my hot tube for over ten years with no problem. Def tech or even Bose will stand up pretty well over time. Mine are out all year round in Massachusetts and the only problem I have is if they fill up with snow. I have to take the grill off and remove the snow, otherwise the sound is not all that good :) I use an outdoor volume control, since they went in before wireless control was available. I also have speakers on my deck, so we can listen to the same thing on the deck and at the hot tube, which is pretty well separated from the deck, back in the woods. Honestly, I think that is the simplest long term solution. The wiring is a bit of a pain, but once done, you should never have to worry about it. I ran 12 gauge in ground low voltage wire in PVC out to the hot tube. Depending on where the hot tube is, you can even just mount the speakers on the house and aim them at the hot tube. Yes, you have to punch a hole in the house somewhere, but that is usually not a big deal depending on construction of the house. The bigger problem may be getting the wires from the receiver to the outside wall, again depending on construction. Obviously, I would suggest a wired system.
So, first I think you need to decide if a wired system is possible or not. If so, I think it will be the most reliable. If not, then just say so and see what you get for recommendations on wireless outdoor speakers, driven from your existing inside system.
If you want to go wireless, do you have a weather tight structure nearby with power to house a wireless receiver or is the setup fully exposed? If the weather is cold in the winter, I am not sure you want any significant wireless electronics, like Sonus, in an unheated structure.
Just some things to think about.