Everyone you ask will tell you something different.No doubt this is a true statement, however I do not agree with this statement,
In the end you will still have to make a best guess judgement.As far as voicing, timbre matching, or whatever name you give to it, here is a good read about matching front and rear, and the same is true matching front and center.
http://www.soundandvision.com/content/matching-front-and-surround-speakers
The best matching center to your original 683's would be the original HTM61, as it was designed by B&W to be used with the 683's, if you can find one and don't mind buying used. Next would be the current HTM61, and then last, a HTM62.
As previously stated, output will be balanced using the calibration function of your processor.