That is coil width. Im pretty sure the MX coil is way shorter not to mention the XXX uses 2 coils(split coil design)so does the mx have more excursion the xxx since it has a 4" coil and the xxx only has a 3"
The mx's coil is 4" around, the xxx is 3" around. But the xxx has a longer coil. The xmax on the mx is 44mm total, where the xxx is 108mm total. The xxx and mx use the same basket but not the same motors. The xxx uses your classic magnet setup,where the mx has neo. That's where you will run in to problem's. The xxx is around 80lbs each with no enclosure. Your car is not going to take that much extra weight! The last dual xxx setup I did was in a 2007 chevy trailblazer and it bottomed out the shocks in it! By the time you dosince the xxx is better than why does the mx have a longer coil
Both new and old XXX would get rid of the MX.. But the MX is still a decent sub...The newer XXX has tons of design flaws, and for the price I personally wouldnt get one...
Also a RD Audio Alpha will get loud on 2k and sound great doing it...One of the best overall drivers I have used..