The stock speakers are chosen nicely for the enclosures they are in. I was surprised at the bass response they had for stock speakers. I did not want to change them without knowing the precise data for the mounting areas.
Every time I have arbitrarily replaced them, I lost bass response, and had to install a sub enclosure. I have a decent 50w X 50w VR3 deck installed, which is SO strange. The time gets set automatically, but, when we set the clocks back an hour? The unit goes ahead 3hours and 20 minutes. No matter WHAT time I set it to, it will always go 3HR 20M ahead of the current time. When the clocks go forward an hour, it goes back to the right time all in its own. Now, tell me THAT'S not freekin' weird...
There is a factory sub-woofer you can install under the driver seat. So, if you install aftermarket speakers, and lose some bass, then you can get the one that's designed for under the seat. I was looking for one myself, but I left it all alone and kept with the OE speakers. They're really not all that bad.