TOPIC:

Does Dirac filter apply to all 4 outputs when turned on or can it be turned on for only 2 outputs and be off for the other 2 (eg. for headphone amp connected output 3 and 4)?

Dirac filters are applied to the left inputs, which are then used to create the signals for all outputs as per the signal routing. So with Dirac turned on you can't have it applied to two outputs but not the other two. You can though easily toggle Dirac on/off.

But you could setup preset 1 to only output on 1 and 2 and present 2 to output on 3 and 4. Then only set a dirac filter for preset 1 and not for 2. This effectively means there will not be a dirac filter for the preset set to your headphone output.

Doing the above still means a button press is needed to go between modes, so doing what the OP had in mind and turning Dirac on/off would basically be equally efficient in use but would take less time to setup and would leave more spare setups that may come in handy.

No, it applies to all outputs available, also digital. You have to create a preset without any filters to listen to headphones. Dirac has to be turned off manually. No way to turn it off via preset. That's what I did with my headphone amp.
Greets
Chris