I did an overhaul on my car audio a few months back. I didn't go with (but will possibly add later) a sub, but just wanted something better than the paper-coned speakers and a new "head unit" (radio)...
I swapped out the speakers first since they arrived first, and honestly, I didn't notice a LOT of difference. It was better, but not great.
The biggest improvement came when I swapped out the head unit for a better one. WAY less distortion on the low end and at higher volumes.
So then I thought, did I need to swap out the speakers at all???? So one Saturday, I put the factory speakers back in. Nope. They are junk compared to the new speakers. I guess when you have junk speakers, you can put in a junk radio at the factory. Then no speaker sounds really good. But if you replace the junk head unit with a really good one, it will make good speakers sound great, and the junk speakers at still junk.
With that said, the sound was still better with the factory speakers and new head unit over anything with the factory head unit. So the biggest improvement should come from a new head unit, unless you are lucky and the head unit in your vehicle is top notch.
