1. Go to AVR`s setup menu
2. Select “Speakers”, enter “Amp Assign”
3. Select “11.1ch” for “Assign Mode” setting
4. Select “Front” for “Preout” setting. The other option is “Height”.
Connect an external 2CH amplifier to the Front L and R preouts, connect the rest of speakers (9.0) to the remaining receiver`s speaker terminals (C, SL, SR, SLB, SRB and 4 height channels) for a full 7.1.4 speaker configuration.
With the AVR-X3600H configured to reroute the main front L/R internal amplifiers, the preamp is physically disconnected from the power amp. This allows the preamp stage full capability of delivering up to 4Vrms unclipped when connected to external amplification as can be seen in the following output vs distortion plot courtesy of Sound United Engineering.
Editorial Note about Front L/R Channels Preamp Disconnect
Front L/R channel preamp disconnect ONLY happens in a 11CH speaker configuration. If you have simply 9.1 channel configuration (5.1.4) or less, and you want to drive front L/R speakers out of external amplifier, or L/R/C, or L/R/C/SL/SR speakers out of external 5 channel amplifiers, internal amplifiers will not be disconnected. There is no selecting or sensing mechanism to allow that unless you activate 11.1 channel Amp Assign mode and select Front L and R for preouts.