The XXX will have lower distortion at just about ANY output level. Remember, what woofer you LIKE, and which has less distortion aren't necessarily the same thing, if your ego is willing to concede that, which it probably isn't.
For ANY musical application, given even remotely similar displacement levels, I'll always take the driver with lower BL distortion. Remember Ryan, BL distortion = 70%+ of ALL loudspeaker distortion. Given that we are comparing a standard overhung driver with a 22mm xmax to an XBL^2 driver with 32mm of xmax, it's not difficult to figure out which drivers comes out ahead in terms of distortion or output.
As far as why I mention displacement difference, even if the MX DID use a flat BL technology for the motor and coil design, it still has about 50% less displacement than the XXX, so it would still end up with higher overall distortion at most output levels. I thought that was fairly obvious.