Sean, a recommendation to use 12 ga is actually a bit conservative for the lengths usually used in home setups. The basic principle involved is that the resistance of the speaker wire shouldn't be too great compared to the impedance of the speakers. Resistance not over 5% of the speaker impedance is a rule often used. 16 ga has a resistance of about .008 ohm per foot and 12 ga about .003. As you can see, 20 feet of 16 ga(which I use) would have a resistance of only about .16 ohm and this gauge should be quite adequate in your size room for your M50s and QS4s. There's nothing wrong with going with a thicker gauge, of course, except that it's a bit more expensive and harder to handle.
