ADI-2 Pro has 2 DSPs, signal processing units which include volume control, for it’s outputs.

Both DSPs are occupied when using the balanced ‘phones option, so non left for SPDIF volume control.

This gives you the following options:

• Change between headphones and speakers, e.g. by switching “Presets” for each.
This can be done by a single button, see below.

If you really need ‘phones and speakers at the same time:

• Connect the ‘phones single ended.
When using a 4-wire cable (instead of the common single ended 3-wire cable) I promise you won’t hear a difference.

• Connect the speakers to an external DAC with volume control on ADI-2’s SPDIF out.
These come in various flavors and prices.

• Switch to ADI-2/4 Pro SE, which has a dedicated balanced headphones output that uses just one DSP, leaving the other one for analog and/or digital volume control.

How to toggle through 2 or more Setups with one single button:
https://forum.rme-audio.de/viewtopic.ph … 94#p193294

You can store and use a “Setup” for each configuration.
If you enable “ Load Volume with Setup” you get what you want.
Read manual page 29 (DAC), 30 (Pro) or 29 (2/4 Pro SE).
Loading certain “Setups” can be assigned to a button push on the device or remote.
Read manual page 26 (DAC), 28 (Pro) or 30 (2/4 Pro SE).
Watch out:
The  “Remap key” configuration is part of each individual “Setup” and needs to be set for each individual “Setup”.
This can be used to toggle through 2 or more Setups with one single button.

Works like this:
As the button assignment isn’t global, but part of each “Setup”, you can even have different assignments to the same button in each Setup - a nice feature that we can use for the following:
Choose ONE single button that you usually don’t need.
In Setup 1 assign: “load Setup 2”
In Setup 2 assign: “load Setup 1”
Now this single button toggles between both Setups.

This method can be extended to load even more Setups if needed, like:
In Setup 1 assign: “load Setup 2”
In Setup 2 assign: “load Setup 3”
In Setup 3 assign: “load Setup 4”
In Setup 4 assign: “load Setup 1”
Now the single button circles through Setup 1-4, clever, isn’t it?!

As for loading Setups ADI-2 asks for confirmation, a double-push is needed to execute the load.
Holding the button more than 1s, the button‘s original function is available.
Therefore I typically assign the SETUP button to load Setups – sic!

Yes, it does sound a bit confusing. Why do you need sound from both the balanced headphone output and your speakers at the same time?

You can have separate volume settings for your main out and your headphones if you drop the requirement of outputting sound to them at the same time. You can toggle between your digitally-connected speakers and your balanced headphones by holding Volume dial (or VOL button on the remote) for a second. In my setup this works with the following settings:

Dig. Out Source = Main Out
Balanced Ph. Mode = Auto
Toggle Ph./Line = Ph 3/4
Mute Line vs. Phones = 1/2 & 3/4

It's also can be useful to refer to chapter 14.1.4 in the manual.