If you can I suggest moving the m2s away from the corners into the room a couple of feet. Sounds like boundary effect to me.

I have heard of people putting a resistor in between the tweeter and woofer binding posts, then connecting the speaker wires to the tweeter posts. This attenuates the voltage/current going to the woofer. I've always thought an eq would be a much smarter way of doing things though.

Good luck.