What is considered an upgrade for my car stereo?

I have a 2005 Ford Focus SES. It came with all stock audio systems. I already installed a new Pioneer bluetooth double din receiver in the car, but want a little more bass and volume. I don't want to go as far as a sub, but perhaps just some bigger nicer speakers than stock. I can't find what the stock speakers are though. What should I look for that would be an upgrade? But not totally crazy or expensive. Just want to replace the 4 in-door speakers.

You have 6x8" speakers in all four locations. You can't just increase the size of the speaker, as they would no longer mount into the door/door panel would no longer fit over them/window may hit the magnet.

Replacing the factory speakers with aftermarket generally yield better overall sound, but the actual bass output is generally not improved at all. In fact, it is often time degraded, depending on what factory system you have versus what you're installing. There's plenty of great options out there. People could post up dozens of decent recommendations. "oh you need to buy Kicker" "Oh man I have these Rockford Fosgates" etc etc Whatever. Whatever. Point being, a lot of companies make decent speakers for under $100. You need to get out there and listen to as many as you can and buy whatever sounds best TO YOU.

And as I hinted at, really think about adding a small subwoofer in there. It'll handle all the low end you want, plus you can set the HPF (high pass filter) on your Pioneer to pull bass out of your door speakers. This will allow them to play cleaner and louder.