Jim, the stripped ends of the wire don't have to stay at 12ga. It's the overall resistance of the entire length of wire that's relevant, not the resistance at a particular point; this isn't analogous to a 4-lane highway suddenly turning into a 8-foot wide dirt road. You can trim enough strands off the ends of the wires so that they fit, and the overall resistance won't be significantly affected.

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Enjoy the music, not the equipment.