Xmax is the amount of linear excursion a driver has one one way in or out. Normally its measured in mm. Generally Xmax is considered the usable limits of a drivers range. And what is considered clean or linear output.
Its measured two ways. One, is to take the length of the magnet gap and divide by two. This is usually more conservative. The other is to measure when the value BL is at 70% of the of its original strength. Or when the motor (voice coil and magnet ) working together are at 70 percent of the total strength. Xmech on the other hand is how far a driver can move total one way. So when the driver runs out of suspension travel and can't move anymore and the driver is at or will bottom if pushed anymore . Xmech is measured in mm.
Excursion is usually a term meaning total travel of the driver both ways. and is often measured in inches.
You can't tell the loudness or quality of bass from only how far it can move in and out. of how far it will travel its not that simple.