The easiest way is to buy the same brand and in the same line up. If you have the Boston Acoustics...use other Boston Acoustics. Or scrap the center and just get a whole new matching set.
If you go for the replacement gig, what is your budget?
I wouldn't try to match a "similar brand" to your CRC center. Either go get a matched "timbre" set of BAs for your fronts or scrap the BA center and buy three speakers of the same brand, 2 fronts and a "matching" center.
At that point, it's nice to have matching "timbre" rears but not critical. You could use just about anything else for rears but you're going to want to keep similar sounding speakers there.
As Ed said, (and Ed helped talk me into my HT set-up I might add), you probably don't want to put a really bright forward set of rears in with laid back fronts or vice versa.