Passive low frequency crossovers for high power applications are impractically expensive to build, requiring enormous inductor values wound with heavy gauge wire. You'll spend more designing a passive network than the amp is worth. For low frequency crossovers it's almost always less expensive to simply buy more amplifiers and use active crossovers....
The ideal solution is an active adjustable signal processor and multi-way crossover with good phase and equalization controls... When properly configured (requires sound-tests with mic), these systems can overcome the vast majority of issues that plague more crude crossover implementations.. An example of such a product would be something like a dbx driverack 260.