ADI-2 DAC has a very effective volume control that can be IR remote operated.
You can connect external amps to ADI-2’s variable output.

I even would suggest to try ADI-2 DAC‘s very capable headphone amps.
In fact there are two built in, one for regular ‘phones, a second one for IEM’s.

Both, XLR and RCA outs carry the same signal, just that XLR is 6 dB hotter by design.

Both, together, can be set to fixed or variable.
The term “Line Out” does not mean fixed level, it’s opposed to headphones- (or speakeramp-out).

1200 mW is more than ten times what you ever use even with power hungry planar magnetic phones.

Example, Hifiman Susvara 60 Ohm, 83 dB/mW, one of the hardest to drive ‘phones exist:
1200 mW drives it to 114 dB SPL.
If you assume very dynamic music, with 20 dB peak to average ratio (most music has much less), you still get an average, “permanent” level of:
114–20 = 94 dB SPL.

According to NIOSH (see below) you may listen at 94 dB SPL for a maximum time of one hour per day, if you want to avoid permanent hearing damage.
The rest of the day you would need to stay in perfect silence to recover.

Practically, even with hard to drive phones, I usually don’t exceed 10 mW, even with dynamic classical music.
This is the reality, the proof of the pudding.

And most planar magnetic phones have 10 or more dB higher sensitivity than Susvara, so with 1200 mW you reach into the SPLs of gunshots.
You don’t go to a shooting range without ear protection, do you?

Give it a try (ADI-2 ‘phones amp, not the shooting range) and you will agree.

BTW: ADI-2’s meters can show the output level, so you always see how much unused power is left.

thats whats missing in the manual. or I didnt read it carefully enough. thx for this sentence. now its clear.

I didnt want to start a discussion about the suggested power to drive a Planar driver. sorry for that and thanks for clarification

I dont need the extra features for the adi-2/4 at all, so for me the difference in price gives a lot of money for a dedicated headphone amp. sad that RME did not put that new (and now with balanced connection) amp into the smaller series. market for all in one headphone amps / dac / preamp is growing fast I think, and I would like to have a all in one solution.....argl....maybe I give it a try. thx to everyone for answering my questions.

how much is the exact power output of the adi 2/4 pro se headphone amp in balanced mode? rme sais 4w, I assume thats for 16 Ohms....then 2w for 32 Ohms? and how much at 300 Ohms? are there any more detailed numbers ?

another question: " Minimale Lastimpedanz bei Hi-Power @ +25 dBu: 40 Ohm"
means I cant use 16 and 32 Ohms headphones on balanced phones with high power?

@KaiS: you are my man ! Thx again.....helps a lot.