Should i buy speakers or not? 96 Bose system currently

To start off i have a 1996 with Delco Bose CD player. There are two speakers in the front doors and two by the back seats. There is also the Bose subwoofer in the driver's side hatch.

I've been reading that the Bose speakers are 2 ohms and most aftermarket are 4 ohms. So if i put lets say top of the line pioneers in, the speakers would sound quieter is that true?

I kinda like the stock cd player so i wanted to keep it. I just want the system to sound louder. Also the rear subwoofer is there a company that makes a direct replacement that is self-powered and that would be louder? If anything i would put a little amp in between the sub and the headunit to power it a little more.

Another question i have is, are all the speakers in that system 6.5inch? According to a website i went to it showed all my speakers being 6.5s

Thanks guys

The speakers are 6.5s. I plan on going with Pioneer 6.5" 3-way speakers with my Bose system. I like the Bose, but the components are just old.