As maxflinn said, the fronts and centres should match for a smooth front to your sound (panning and not emphasising a certian speaker) so either RXLCR and RX6s or go for KEF floorstanders and hope they match ok. Sub- aslong as your happy it fills the room then your fine, a more pricey sub will just give you more scale if thats what your setup requires! Rears likewise arent as important as matching the fronts, just set them at the right level, dont emphasise difference by having their level too high.