You need to remember the rule of 10's when it comes to loudness.
I don't know how many watts the M1 puts out, but in order to be 2X as loud, you need 10X the wattage. IE: 1W, to be 2X as loud, you need 10W. Apparent volume is a different discussion.
I've put in jobs with SP12F's, but I put in 4 to balance the load down to 8 ohms typically, then work the values from there (# vs. seen impedence).