It is the distortion that most often "blows" speakers. Factory headunits are usually underpowered and have a lot of distortion.

Here is how it goes down...
You turn up the volume cause ther isn't enough power going to the speakers to make them sound good. The more you turn it up the more you are pushing the factory headunit amp to its limits. This causes really bad distortion and BANG - blown speakers.

I am currently running factory headunit and speakers with an Alpine CD changer. All my **** was blown by its previous owner and I got no money to do it right so I am waiting a while.