hi grdunn ..
ive actually got rx6 fronts with a br centre , sub and sats , i did have br5 fronts initially ..
i cant say i found any issues after replacing the br5s with the rx6s , ive used my yamaha amps auto set-up with the microphone , and it all sounds good to me , although ive upped the level of the centre a touch on my yam , not because i felt i had to , but because i like good loud dialogue for movies ..
i dont know if your rs5s would match with a br/bx centre ? some of the more experienced forumites might have an opinion there , although the usual advice of demoing will probably be given
..
have you considered a 2nd hand rs centre ? that will obviously match , and it would probably cost less than a new br/bx centre too ..
btw the rears are apparently easier to match , its the front three that are said to be the most important , and any decent sub will fit the bill afaik ..