How to match HT speakers with existing speakers?

Maybe find out what brand of drivers are in there;look for same drivers in another brand?? HT can still be satisfying with mismatched drivers.--Depends on how much you want to spend to achieve what you're looking for;as you probably already know.

The rears are not as important as the center channel, depending of course on what type of surround decoding you are using. I'd look for another Celestion to match your A-1's. I'd contact Celestion on the matter and see what they say. Ideally you would use all Celestion in your system or sell the Celestions and start from scratch if that is not practical.