The sealed box uses the 12.3 driver while the PB uses the newer 12.4 driver. Besides an upgraded driver, it uses 3 proprietary ports (not off the shelf PSP ports that they used to use). More materials in both MDF as well as veneer. A more expensive grill. More labor.
Build your own sub using your high quality parts and complete it to the fit and finish that SVS has. It's not easy nor quick.