For 100 watts a channel, I'd run 16 gauge wiring just to be comfortable.
Although it's doubtful you'd notice the difference vs. 18 gauge - I'm pretty sure the stock wiring is smaller than that. Like 20-22 gauge, iirc . . . .
I find 16 ga. to be a pretty good compromise 'cause it's inexpensive and you can find it with relatively thin insulation so you can run it under carpet without a lump.