Zak, I use 16gauge lampcord for all my runs(none longer than about 30'). You sort of did it backwards; if you were going to use both 18ga and 12 ga, the 18ga should have gone on the closer M60s. An excellent explanation of speaker wire in general by a veteran audio professional, with a table of suggested gauges, is found here and is well-worth studying.

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Enjoy the music, not the equipment.