Years ago I wanted more bass in my swing so I replaced the rear speakers that exist in the seatbelt mounts with good quality ones for clarity, then put and unamplified (just wired it to one of the rear speakers) subwoofer in the boot. It gave out tons of bass and the car ooked original. The rear window used to shake!

Also it meant the car still looked original.

Now that I am older.....I just put in decent 10cm speakers where the existing ones are and that is usually fine, age