Here's the thing, your 18 guage cable might do the trick, so if you don't want to swap it out, more power to you. A 50 foot run is fairly long though, I would personally use 12 guage for them. In fact I do. I bought two 100 foot rolls of 12 guage wire from Monprice for $24 bucks each, how can you beat that? Down at my local HH Greggs they only sold Monster cable, it was $125 bucks for a 50 foot roll of 12 guage.
If you go through mono price you can purchase the thick stuff so cheap that you might as well get it 'just in case'. Now if there was say $100 dollars difference between the two guages then I'd say buy what you need, but at $24 bucks... come on, you can't beat that! And its plenty flexable, so don't worry about that.
Welcome, I can't wait to hear your impressions once you have everything set up. Do you already have a sub?
