I would recommend against upgrading the amp while retaining the stock speakers. Purchase a couple of sets of component speakers. These sets come with their own crossovers.

If you are replacing the speakers, then an additional crossover would not be necessary. Your amplifier will have a high-pass/low-pass setting and the component crossover takes care of the rest.

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2004 BMW X5 3.0i - Mine
2011 BMW 328i xDrive - Hers
2002 BMW 325xi - waiting for a new owner