Meiwan wrote: If you are playing redbook cds mainly, the centre/rear channels will just be duplicates of the 2 main channels.
Not so. There are many systems that will extract ambiance information for surround and interpolate a center and these include the ubiquitous Dolby and dts processes. However, the OP is advised to try them before buying since they vary considerably in their capabilities and configurabilities.
Kal