For 10-15W, unless you have very long runs, 18 awg is probably fine. Step up a size or two if you think it looks too puny...

If you want to protect the speakers from deep bass, get "bass blockers" from your friendly car audio store. They're essentially just capacitors that you put inline on the speaker wire, acting as a 6db/octave high pass filter. I don't recall the formula for roll-off frequency vs capcitance and speaker impedance... Shouldn't be to hard to find on the net (or someone might have it handy) if you wnat to buy your own caps instead of buying "premade" (ie attached pieces of wires to the leads of the cap and placed in shrink tubing...)

/Michael