California T sound system upgrade

Hi guys
So I just got a 2016 California T and I love the car other than the sound system.

I have the base systems not the HI FI one.

Putting the volume in max would still not be more than average on my Mercedes.

Anyhow looking to do a small upgrade, wanted to get some feedback and or suggestions.

Not looking to spend too much money on this one.

This is what I found out.

Recommended by few people is to add a 4 channel 400 Watts amp and send 2 channels to the front oem speakers.
Like - JL Audio XD400/4v2 (which is small and can fit inside dash maybe)

The other 2 channels i will is in a 10 inch JL audio sub woofer -
10TW1-4: 10-inch (250 mm) Subwoofer Driver, 4 Ω

Any thoughts, or suggestions....

 

Also, anyone has an idea where is the oem amplifier is located ?