A good point, but there's a few things to consider about xmax predictions. The first is that your box program is assuming that the driver is receiving 150 watts pure sine at 82Hz, and if you were pushing 150w with an 82Hz fundamental- E2, almost right on the button- perhaps half of that wattage would actually be contained in the fundamental, the rest would be harmonics, so xmax wouldn't be approached. Dealing with musical program as opposed to pure sine waves you can expect that usable linear xmax predictions usually run about 3dB low. Also, xmax is based on the travel of the voice coil within the magnetic field of the motor, and is very different from xmech, the actual physical limit of the cone/coil travel range. While the xmax of the D12LF is 4.8mm, the xmech is 26.9mm. That allows for a lot of travel beyond xmax before coil slap can occur. Even more relevent is the xmax spec itself, which is almost arbitrarily arrived at. For instance, technically an Alpha 8 has a zero xmax. I regularly chat with a senior project engineer at Eminence who recently told me that they are now starting to use Klippel analysis of their drivers, which measures the driver in use and calculates the point at which a 10% distortion figure is reached, a far more useful spec than xmax in predicting driver performance. I wouldn't be surprised to see this spec become an industry wide standard that will eventually supplant xmax entirely.