That's a good question and it deserves a complete answer.
Speakers are omni-directional at low frequencies and highly directional at high frequencies. At low frequencies, the interference between the speaker's direct and rear wall waves creates re-enforcement commonly heard as a "boom". As the frequency increases, the phase of the reflected sound lags the direct creating cancellations.
At some frequency, the reflection will be delayed so much as to be in opposite phase relative to the direct sound creating a rather significant first order dip in the frequency response. At 5", the dip will take place at about 575Hz.
If you could move the speakers 3.6 feet away from the wall, the first order interference dip would occur below the lower cut-off frequency of 80Hz. Your problem would then be solved.
You aren't that fortunate so you have to resort to a different scheme; locate the speaker as close to the wall as possible. This will decrease the time delay of the reflection relative to the direct sound. This moves the interference problem to a higher frequency where the speaker's own directivity decreases the rearward radiation. Note that this will boost lower frequencies but these will be cut off from the M2s anyway because they are being sourced by the sub. Alternatively, the bass boost may be pleasant and more preferable than the sub for some styles of music.
The drawback to positioning the speakers closer to the rear wall is reduced sound-stage depth. Since you only have 5" to work with, sound-stage depth may already be compromised.
So, armed with this knowledge, you will now have to experiment to achieve the most appealing reproduction for your ears.
And like Randy says, toe them in.
