Jack, I'll do the 12 gauge to 18 gauge question and the answer is no problem. This isn't analogous to a 4-lane highway suddenly becoming a 10 foot wide dirt road. It's the overall resistance of the total cable length that matters, and for example, a short length of 18 gauge might result in the equivalent of an overall 12.4 gauge, which is still heavier than needed unless the run is on the order of 50 feet. Also, as has been mentioned, inexpensive cable is available in colors other than clear.

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Enjoy the music, not the equipment.