Use the same speaker across the front or buy ones that are voiced the same. My RX centre sounds very good and has decent bass compared with my RX6 L/R floorstanders. Previously I had Ruark Talisman 11 L/R with their own centre speaker and it was not a decent match and changing to the RX trio made that all to obvious as now its seemless. If you cant get that then phantom mode works well enough as long as you dont have anyone off axis watching the film.
Paul
Onkyo 875, Monitor Audio RX6 GW,Monitor audio RX centre GW,Monitor Audio BFX rears, REL QUAKE,Arcam CD73t CD ,Russ Andrews Yello mains cabling & PBJ interconnects,Sterling Black Mamba speaker cable.