How do you improve rear speaker sound reproduction on an Audi A4 18T 2000 model without changing the head system?

I am in the process of figuring out how to replace my rear speakers in an Audi A4 2001.

So far I have figured out that the cover is attached to the plate. I am going to remove the covers underneath to remove the wires. There is a little cup that cover wires for the speaker, you can remove that one pretty easy. I will update with more details on the project goes.

If you put a crossover to take out the lows below around 70 to 90 Hz, it will remove a lot of distortion.