NO, 6x9 speakers will not fit unless you carve up the rear deck and use aftermarket grills.

No aftermarket speaker will be plug and play. Crutchfield sells an adapter plate that accepts 5 1/4" speakers. I used an Infinity speaker (6822CF) that was marketed as both 5x7 or 6x8 and then used my old speakers to serve as the adapter plate after I cut the basket off.

I did a writeup on how I dealt with mine on all three of my 850 sedans that has some OK pictures:

http://www.matthewsvolvosite.com/forums/viewtopic.php?t=8402

From the outside the system looks factory - it just sounds a heck of a lot better.

...Lee