audio upgrade

[kennethvd1997]

I am looking between car audio equipment and would like to get some information about how I best apply it.

I have a bmw z4 e85 with 10 speakers. 2x OEM tweaters, 2x oem 16.5cm speakers at the feet, 4x mid range (2x in the door and x2 behind the seat) 30w rms / 60w peak 4ohm and 2x oem mid range behind the head rest.
These are all connected in the head unit without an amp. I have a subwoofer installed in the trunk (KSC-W1200B) 300w rms / 1200w peak. I've had a short in the amp that came with that sub, so I need a new amp anyway.

Now my problem is if I turn the volume up a bit louder than average it quickly becomes distorded and krackles.
Which speakers should I replace and what should I connect to an amplifier and what should i leave in the head unit?
And should I take a separate amplifier for the subwoofer or do I look for 5-channel amplifiers?

Thanks for the help.

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