Kris, the overall resistance in the entire path of wire, etc. is what counts, not the resistance at some small point along the way. The added resistance of the wall plates is over such a short distance as to be inconsequential. The few feet of 14ga going to the speakers doesn't decrease the overall resistance very much as compared to using 16ga all the way, but if you have it, of course use it.

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Enjoy the music, not the equipment.