I was told, in general, that keeping the total displacement of the driver (SD x xmax x 2)to less than 5% of the total air volume keeps ASD effects mitigated. I believe there is a more specific forumla somewhere, Ricci mentioned it. I'm sure it is one of those that I knew at one time but forgot, like a lot of things. In any case, I'm just not used to modeling anything like the LMS 5400 in a sealed enclosure. I'm never afraid to build a big box. Since I always build sealed enclosures to a Qtc of around 0.5, thinking about airspring distortion was soemthing I never really had to think about. Not that it is anything major anyway... It is just that the box required to reach a 0.5 Qtc for the LMS 5400 is so small and the xmax so large, it was just something that caught my attention. You obviously don't want to go too big since you lose power handling; you also have to somewhat consider the possibility of subjective sounds of box Qtc, although I'm not quite so sure one can hear the difference if I were to design a box for 0.7 or 0.5. Regardless, if sound quality is the only objective, which in this case it is, you may as well make the box 'big'. Then we also take into consideration that Qts of the driver can rise about 20% when it gets really warm... So when you consider ASD, Qtc, and warming voice coils, it amounts to designing the box to be around Qtc of 0.5 as a general goal, in my opinion... The only other thing to consider are box modes less than 200Hz. The maximum dimension, the internal height in this case is the longest dimension = 38.85", so not an issue.