You can run your M2s in the large mode but the small woofer/cabinet combination will be limited in it’s low frequency output anyway so I am not sure this is a solution. It would seem like the real solution is a steeper x-over on the sub-woofer. Because of the limited low frequency output of the M2s running the receiver on small makes a lot of sense (the speaker is not doing a lot below that anyway). Setting the sub x-over to 80 Hz should not produce any directional frequencies and hence should not be position sensitive. An electronic x-over in the line to the sub seems like the ultimate answer for your system. Other than this, M3s or M22s up front would give you a bit more low frequency information and allow you to run the system on large and lower the x-over frequency setting on your sub.
