If I recall correctly, labelling one of the Cyrus amp's inputs as 'AV Direct' will cause that input to become a unity gain/bypass input for just this purpose.
EDIT: Ah yes, got it now:
Connecting a Surround Sound decoder (other brand)
If your system includes a multi-channel surround sound decoder (but not a Cyrus model), it may be connected through your amplifier to drive the front channel speakers.
In this case, connect a stereo phono interconnect from the front channel outputs of the surround decoder to one of the Analogue Inputs of the amplifier. For this system to function correctly, the ‘AV Direct’ input name must be set for the input used (see ‘Input names’ in the ‘Setup menu’ section).
When set up in this way, the amplifier will automatically lock its volume setting to a fixed level to match the output of the surround decoder.
CAUTION: The ‘AV Direct’ input name should only be used for surround decoders that include a volume control, as the volume control of your amplifier will be fixed at a very high level.