The Cambridge Audio AV Receivers have three settings for the pre-outs:
'Normal' sends the same signal that the amplifiers are receiving to the pre-outs.
'Ext 2.1' mutes the front left and right amplifiers and allows for that signal to be sent to an external amplifier such as the 851A/851W operating with a fixed input. This allows the AVR to run cooler.
'Pre-out' means that every amplifier (speaker outputs) in the receiver is switched (muted) off and it operates exclusively as an AV processor and will need external power amplifiers or active speakers to function. This allows the AVR to run even cooler.