Replacing std speakers, not bose, to better ones?

[NightOwl]

I've been trying to search for info about replacing std speakers in a A8 D3 with std speaker system, and there seems to be nothing. The reason to replace is that they sound like crap, so I hardly use the sound system at all since I bought the car over five years ago. Does anyone know the size of the speakers, and will, lets say a 6,5" speaker fit with the bracket in the door if the original one is 6,5"? May bose speakers from at D3 fit, assuming they sound better? I'm not thinking about replacing the amps or anything else, just the speakers.