Zub, Mr. Russell doesn't show distances longer than 50 feet on the chart, but you'll notice that the entry for 6ohms for 14ga would have been 20-40-(60) feet. So 14 ga would be okay for your 45-50 foot run. Don't worry about trying to cut this too fine; a few feet doesn't make a significant difference.

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Enjoy the music, not the equipment.