How many ohms are the factory non-JBL speakers in a 2009 Corolla S 6 Speaker sysyem and does yours sound better with upgraded speakers?

[borninabarn]

We just purchased a 2009 Toyota Corolla S and it has the NON-JBL OEM 6 speaker stereo system in it.It shows that it has 6.5"speakers in the front doors,tweeters upper front doors and 6 x 9 speakers in the rear.How many ohms are the factory speakers in this car? And if any of you replaced the stock speakers and kept the factory radio does the system sound much better? I think the rear stock speakers would sound better with a co-axil or three way than just the hum of the stock one way they put in.Let me know before I decide to tear out the back area to replace them and find out I should have left them alone.Thanks,Born