[lkchris]

Your Porsche amp is going to send just bass frequencies to the door speakers because it expects them to be subwoofers (only). This is what your replacement speakers should be as well.

Are you going to be able to find an aftermarket amp with integral crossover to power subwoofers? Or will you fit an additional crossover?

The design from Porsche has two channels for the midrange rear speakers, two channels for the dash speakers, which are midrange plus tweeter with the crossover circuit at the speaker, and two channels from the amp internal crossover to the door subwoofers. Total = 6X40. Most Boxsters come initially with 4X40 amps, missing the rear speaker channels.