Andrew, if your ceiling is about 8', the 8'x10'x12' dimensions are quite good from the standpoint of room modes, so it doesn't appear that they would pile up in the upper bass to a great extent. The boundary effects which John B. described are a second type of room influence, but you may not have enough space in your small room to set them at the suggested distance from the walls. Note also that a similar effect to the one John B. described at 3' from the walls also occurs when the speakers are separated by twice that distance(e.g. 6')and are playing the same sound. A suckout around 95Hz results.
You haven't mentioned what sub you have and usually a 150Hz crossover would be only a last resort if the main speakers were weak below that area, but you can certainly try it and see if the result is a net improvement.
Yet another placement factor that should be mentioned is that the woofer cones on the M3s should if possible be at unequal distances from the three nearest room surfaces, i.e. the floor and two walls.
Finally, as has been mentioned, trading in for M22s might be considered.
