Spinner, welcome. You're going to be enjoying a lot of great sounds soon(assuming that you have good source material). Your 18ga wire will be fine for your use; more isn't necessarily better. For an excellent explanation of speaker wire by a veteran audio professional, which includes a table of suggested gauges, study this . You've already got the wire, it's thinner and easier to handle, so it's "right" for you. Enjoy.

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Enjoy the music, not the equipment.