...also, I could spend a little more if I had to, to get it right (if the HTM 61 is not the way to go).
Ideally you should choose a center from the same manufacturer and series (in the case B&W 600 series) to match your L/R mains. Manufacturers voice match speakers in a given series for multi channel applications. So I would definitely mate the HTM 61 with the 683's.
Surrounds are not as critical to match from same manufacturer as they do not receive nearly as much content in a movie or concert as the center and mains do. Hope this helps. Good luck.