Uograding sound system

Hello, My M3 has Harman Kardon sound system installed. I'm looking to install an amplifier but I'm told original 3 ohm speakers may not properly work with an aftermarket amplifier. Has anyone done this process?

I just completed a total replacement of the audio system in my e46 323ci. The process is fairly straight forward if you bypass the factory amp in the trunk; swap out the door and rear shelf woofers with 4ohm two/three way speakers (6.5"), power those new speakers with a new aftermarket head unit and your done. You can keep all the old wiring too, just don't use the factory amp and head unit any more and you're good.What you loose in aesthetics you gain in pure functionality providing you aren't looking for audiophile quality sound or a stock look in the cockpit.

I just replaced my stock head unit and kept all the original harmon kardon speakers. Even the stock amp. Made a big difference. Oh and I added a 10 inch kicker with a 800watt amp. It bumps.