Want the short answer? Yes! You should definitely upgrade your factory car speakers. It may not be true for all cars, but the factory car speakers sound awful in most four-wheelers.
This is why it is preferable to upgrade the sound system. However, several of us stop here because they don’t have a budget for an amplifier. If you are among these people, I have good news for you.
It is fine to replace factory car speakers without an amplifier. I took my car through numerous modifications. I did so for my clients as well. All my clients are satisfied with the outcome.
While it’s true that an amplifier improves the sound quality, the performance boost isn’t as exciting as it sounds. Moreover, if you want, you can add an amplifier later when you have the budget.
Basically, this sums up the simple fact that it is okay to replace default car speakers. You don’t have to worry about the amplifier.

