Double posting. Please delete the other one.

Can‘t get this to work either with Rear TRS assigned to 3/4: 
• I/O / Phones Out 3/4 / Settings / Rear TRS Source: Ph. 3/4

Plugging phones does only mute Line out 1/2 with any setting under:
• SETUP / Options / Phones / Mute v.Ph x/x

@MC
For ADI-2/4 Pro SE this option should be included IMO.
Additionally I still vote for handling 3/4 the same like 1/2, with a complete set of independent I/O-parameters for front and rear (if Rear TRSs are assigned).

The Options:
• SETUP / Options / Phones / Toggle Ph/Line: Ph 3/4
let’s you manually switch 3/4 front/rear.
• SETUP / Options / SPDIF/Remap Keys / [select button]: Toggle Ph/Line
you can assign a key on the IR Remote.

@MC
Unfortunately there seems to be a bug, the above settings toggles between muting Line Out 1/2 and 3/4 instead of only muting 3/4 and leaving 1/2 alone.

• SETUP / Options / Phones / Toggle Ph/Line: Ph 3/4
and
• SETUP / Options / Phones / Toggle Ph/Line: Ph 1/2+3/4
do exactly the same thing, probably not intended like this?!

OK, I had Dual Phones on, as this fits my personal usecase:
I have 2 headphone amps connected, one at XLRs 1/2, one at Rear TRS 3/4.
I do use front PH 1/2 and 3/4 for further ‘phones.

Switching Dual Phones off doesn’t change the switching behavior into a useful  config:

No way to toggle 3/4 front/rear while keeping 1/2 alive.
Neither from the menu, nor by plugging ‘phones into 3/4.

Working through all the options, I still find this part confusing.

A table in the manual page 32 what “Toggle Ph/Line” option should do what would help, even in conjunction with 1/2 s Volume Dial push and the assign key option.

The dim and fade-up is for security reasons, to give you a chance to pull out the plug if ‘phones are unhealthy loud.

The main problem is, that 3/4 front and 3/4 rear are not handled independently, like is the case with 1/2.

As mentioned here:
https://forum.rme-audio.de/viewtopic.ph … 8#p228518,
https://forum.rme-audio.de/viewtopic.ph … 5#p228535,
1/2 is better suited as monitor section.

Rear TRS to 3/4 are impedance balanced, have the same audio quality and should be used as output to the analog hardware mastering chain.
This is how ADI-2/4 Pro SE is intended to be used.

Point is: the unit was not designed to use all outputs independently. There are only 2 DACs, so obviously many features like EQ per output are limited when you try to use all 4 outputs simultaneously. But this also applies to mutes that are done in the digital domain, and some more.

The TRS 3/4 feature allows a simpler setup of FX mastering loops compared to the ADI-2 Pro, but obviously using phones at the same time then can't fully work - that would require more setup pages, a third DAC etc...or a TotalMix FX equipped multi-channel audio interface. We do have that.