You can use aftermarket speakers with the stock amp (I am doing it currently). You may not be satisfied with the results. I have not replaced the mids, but the sound seems ok. I have a single 10 installed. I had to patch in with a loc. Check in the Mobile Electronics Forum.

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