First off welcome to the forum!
So if right now you have the rs-42 as your surround back channels, I would suggest moving those to your sides to your surround channels. Then with the size of your front stage you could probably save a buck or two and go with the rb-51 if you can have them out from the wall a bit since they are rear ported. If they need to be flush I would go with the rb-61 as they are front ported and flush mount will not affect sound at all. I def would not even consider the rb-81 as those would probably out do your rf-62 in all reality. I would also try to find a used or new if you can but stay in your generation of reference speakers if at all possible. Good luck with your search!
Home Theater :
RF-63 (cherry) Fronts
RC-64 (cherry) Center
RS-62 (black) Sides
RB-61 (black) Rears
DUAL SVS PB13 Ultras (real american cherry)
Pioneer Kuro 50"
2 Channel Setup:
Klipsch 1985 Cornwalls in Walnut
Extra stuff laying around: wf-35's, rf-52's, 42ii theater, velodyne optimum 8, mini-x,