Blown driver

I left my stereo on for my house keeper while I went to work, bad plan. I came home to a blown driver in my one of my B&W 803s speakers. I ordered one & one just in case. My question is do I replace all four or not. They aren't cheap. The speakers are getting a little age on them & so am I.  My amp is in excellent condition & rated at 375 WPC. I've played them very loud with no problems. Thoughts?