I am not entirely certain what audio system your car came with, but:
The P38 has separate amplifiers per door, they are built into the door itself and are fairly low power but some guy once measured the subwoofer amp (which is largely built around the same chip) to have 15watt rms (real electrical) output power before clipping. For audio that is quite a lot but it depends on the efficiency of the speakers.
Having amplifiers per door (and sub) is both an advantage and disadvantage. Replacing the head unit won't work easily without a high to low converter to make sure you do not over drive the amps or use a radio with amp outputs. Then again, the sub has it's own audio feed already so replacing the amp for a more powerful one is quite easy (you only need to run a power feed).
So replacing door speakers is relatively easy and if you do not need more power it could be done quite quickly. I have found the mid speaker to be the most difficult to find. (a fitting one). The door amp has 2 outputs (it's basicly a stereo amp chip of which both channels are used). One channel drives the low range in the door and the other channel drives mid and high (in the front, the rear doors only have low and mid). iirc the high is filtered by using a cap over the tweeter, thus it get's fed the full spectrum from the mid speaker as well which is normally not what you want in a high end system. Also, again iirc the mid range kicks in at around 200hz. The low speaker also get sub fed.
in short:
replacing the head unit requires converter cables and possibly transformers to feed the door amps.
replacing the rear sub requires a power feed and modifying the audio feed to the aftermarket amp.
replacing the door speakers is straight forward, they just have to fit (or made to fit )
lastly: getting more power to the door speakers is one hell of a job, you would need to run your own speaker cables to each of the doors, run the, in the car to an amp (or amps) feed that amp with audio and power, etc etc.
Personally I think 15 watts of RMS power is quite enough. Maybe not so for the sub but definatly for the doors. If you change the head unit and have an option to filter sub from your front/rear outputs then that is completely sorted as far as I am concerned. More sub power can be beneficial even with replacing the speakers in the cabinet to something more suitable.
Cheers,
-P
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