I forgot you're running it in the wall. My concern there is two-fold: 1. finding sub cable that is CUL-approved for inside the wall and 2. what if you pull on it and end up disconnecting it? I ran my sub cable on the outside because I didn't want to fool around with terminating coax. It was easy enough because I hid it under the trim. You may want to do the same. I think it's more practical because you don't really know where the best place for the sub is until you try multiple locations.
I don't know about the US but Canada's HD does have CUL 12 gauge speaker cable.
You don't need wall plates now but make sure that the speaker cables are covered well so you don't get foreign matter in there.
