Check the stickies...
Speaker Sizes, Harnesses, and Kits for the Mazda6!
Again, consider the Bose system as a package deal. If you replace the speakers you need to replace the amp. If you replace the amp you might as well replace the speakers too. Just replacing one of these is not going to result in desirable sound. It will be different...but not necessarily louder and certainly not worth the price you will have paid.
The only good news about the Bose system is that the head unit puts out 2v pre-out signal for your aftermarket amps. That means you can replace the amps/speakers and leave the head unit for the stock look.