Graeme, keep in mind that a receiver is a combination of a pre-amplifier and an amplifier in one chassis. When a separate external amplifier for one or more speakers is connected to the output of the corresponding pre-amplifier channels, the internal receiver amplifier circuitry for those channels is bypassed and the separate external amplifier takes over the job of powering those speakers. None of the control and processing functions(including for surround sound)of the pre-amplifier section are changed, because they take place before the pre-amplifier outputs(pre-outs).
