before any more bad info hits this thread I'll chime in. SPL is as always still all about displacement, but it's abotu displacment near tuning. At tuning as most ppl know speakers have very little excursion as the port will be loading the speaker very heavily. This means XMAX mean very little as getting to XMAX near tuning is the hard part. In order for a speaker to get moving when playing at the resonance frequency of the box it needs a strong motor and a light cone.. That's why SPL subs have high BL (motor force) and use such light cones, to increase potential displacement at tuning. Lastly, they need to be able to handle alot of power which is why they tend to have bigger coils.
XMAX matters most in sealed enclosures because as frequencies get lower you need to move more air to keep SPL the same. After all it's all about displacing air and if the cone is moving less times per second, it needs to move further to keep displacement even. However, XMAX does matter in SPL subs too. If a sub has low xmax, that means it is losing BL early as the coil leaves the gap. XMAX is defined as the subwoofer losing 30% of it's BL vs it's resting value. BL as I already mentioed is very important for an SPL not, not only the static number at rest, but keeping that BL through the stroke. An XBL^2 SPL sub could actually do very well if someone built one properly.....