Speaker options
It is important to have a good tonal match across the front channels. So stick with a single manufacturer for the front L/R and center. If you are happy with the B&Ws, then buy the matching speakers. If not, then move them to the surround or surround back channels with 5 new speakers.
The sub does NOT need to be the same manufacturer as the speakers. Check out the 10" and 12" models from HSU and SVS which start in the $400 price range.
edit: Mike, thanks for catching the typo.