I'd certainly try the Onkyo AVR before shelling out cash unnecessarily. You can easily make it Bluetooth capable with an inexpensive kit. Monoprice has one here:
https://www.monoprice.com/product?c_id=120&cp_id=10827&cs_id=1082711&p_id=10248&seq=1&format=2
Or save a few $ Here:
https://www.amazon.com/dp/B00DIGAO22/?tag=thewire06-20&linkCode=xm2&ascsubtag=AgEAAAAAAAAAAHckAAAAAHVNPMcAAAAAWVHtLw
Here's a positive review for the AW6500s.
http://hometheaterhifi.com/reviews/speaker/bookshelf/definitive-technology-aw-6500-outdoor-speakers/
They should match the Onkyo AVR quite well. I'd run the AW6500s in Stereo Pairs, especially if there's quite a distance between the pairs. Run one pair from the Front Speaker connections, and the second from Zone 2.
As for the Sub, I'd be tempted to try to use two self powered subs, one with each stereo pair (preferably in the middle). I'd at least go for a 12 inch unit as the bass will be far less evident outdoors. I know there are wireless Sub kits out there, as I have one from Best Buy in the basement. I bought it but never hooked it up, as I always seem to hard wire them. You'd need two different kits or wireless Subs that operate on different frequencies to go wireless.
There are also wireless subs coming on strong, but few are of the outdoor variety. You may want to consider indoor versions, and be prepared to pull them inside. Alternately, you can go with High Level Inputs to each Sub, and then connect to AW6500s in Stereo Pairs. (One downside is there's a lot of disconnecting to bring the Subs inside.)
Do you have a sketch of the layout, or photo we can see for proper Speaker placement?
Just my two bits.
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