Personally, I thought setting the cross over to 80 was too low. In my system, the M3s work better if crossed over at 100-110 Hz.
2x6 brings up a good point. 80Hz is the most commonly recommended crossover point. As mentioned above, as long as you are without a sub, just set your M3s to "large." When you eventually add a sub to your system, set them to "small" and start with an 80Hz crossover. But, don't hesitate to experiment with other crossover points to see if you feel another would sound better.
