Are they installed? If not, is it just the "uninstalled" mount hardware that's rattling or, if they are, something in the wall?

I wouldn't expect M2s to really crank, but if you are connecting M3s with the exact same setup and volume and they're doing fine, something is up. I'm guessing it's not the drivers themselves, because the odds of you getting two faulty speakers is really low......

::::::: No disrespect to Axiom, but my favorite woofer is my yellow lab :::::::