Dear KaiS,
thank you very much for your answer, I would like to add what we specifically plan to minimize the signal paths and sorry we know indeed thousands of parameters of our synthesizers, effects and mastering tools, but are electrically seen absolute zeros and must ask here for apology.
I think it would be good if we could elaborate on what we are trying to do. We want to give up our good old but inflexible DAC (Weiss DAC1) and the AIOs. If we need further interfaces, we can realize this via the mastering processors or an audio interface (outboard). Furthermore we want to avoid DSD and sample rate conversion (Weiss Saracon) or stand-alone players (HQPlayer) as far as possible in the future and switch to linear amplifiers (Benchmark AHB2) as well as buy new speakers.
We did extensive research on the DACs (AudioScienceReview) and took a close look at all measurements and compared them. We then auditioned our personal favorites. With our tendency to switch from Weiss to RME, we also established clear preferences.

1. Routing
What have already come to know is that we appreciate a high performance DAC against audio interfaces for routing, on DAC-level this can be seen as a unique selling proposition. Since we still use other audio interfaces and mastering processors as well as effects in the rack, we must be able to integrate them as specified.

As written, we consider the routing matrix given by RME in the user manual to be complex. We personally cannot see from it which channels / interfaces can be used in parallel or which restrictions exist (sample rates, DSD conversion etc.). In the manual, limitations are mentioned where the DSP obviously reaches its limits - no wonder considering all the features RME has realized here in a great way.

1.1 We want to access the RME DAC via USB - PCM or DSD with any sample rate
1.2 Feed this signal via AES/EBU with 24Bit 96kHz or 192kHz into the effects rack, where it runs through several processors (outboard)
1.3 Pick Up the signal from the effect rack via AES/EBU with the RME DAC and
1.3.1 record it via USB with the DAW and
1.3.2 output it to the analog channels of the RME DAC in parallel

Is this explicitly possible, if not we need to keep the audio interfaces in the DAWs, SRC, DSD conversion et c..
May we politely request if someone could please try this and confirm, and announce restrictions if any, e.g. routing is possible but no sample rate conversion, no DSD conversion, only sample rates up to 192kHz et c..

2. Output Gain
We want to use the RME DAC directly in front of the AHB2 power amplifiers if possible and use the volume control from the RME DAC, is this possible?
Benchmark's power amplifiers (AHB2) cope with the following inputs by switching options:
Low-Gain = 22 dBu (9.8 Vrms), Gain = 9.2 dB
Mid-Gain = 14.2 dBu (4 Vrms), Gain = 17.0 dB
High-Gain = 8.2 dBu (2 Vrms), Gain = 23 dB

We want to use +22dBu here, as recommended for balanced inputs.
The second analog output would therefore not be usable, or we have to use the switching options with hopefully little loss of quality.
However, as written in the answer, if the second analog output is always -6dB less, this will also be futile.

2.1 Y-adapter
We have always avoided splitting cables or using adapters. If it can be confirmed that neither the level nor the signal quality suffers, we are happy to consider this.
We could have a cable made specifically for this purpose.

Dear KaiS,
thank you very much for your help and support in this matter, we run our studio and band privately and cannot afford any mistakes, especially since we have to scrape together the money for it anyway and want to afford a modernization 10 years before we retire.
1. excellent that the routing works. Most of the time we use "only" 96kHz for the studio equipment anyway, unless there is another crooked recording or an absurd audio BluRay in this sample rate needs to be polished.
2. of course we don't listen that loud in the studio, the second RME DAC is for the stage. During a live event we want to achieve about 99 dB SPL at 10m, that's what we designed and dimensioned the system for.
2.1 Absolutely awesome KaiS You speak from the heart! One of us is an absolute Van den Hul freak, hopefully I can convert him a bit with your statement. I only use standard cables, only for analog connections I use Mogami, and even here only the standard series. My personal opinion is, if standard cables change the sound, they are bad cables, like in the case of analog connections, super expensive cables from Vovox, In-Akustik, van den Hul etc. In-Akustik's slogan "Physics versus Voodoo" is still groundbreaking in my opinion.
Only pure silver cables (99.9%) for the loudspeakers from first hand and without bells and whistles - if they were not so expensive - would be fine. If you want, you can have them made by a professional in Germany: Axmann Audio. He is also the only official Benchmark distributor there. Thank God the amps are mounted in our second rack further away, so the cables would be too expensive anyway.

Thank you very much for your eloquent comments and quick answers based on your many years of studio and mastering experience.

For longer distances >10m, but only then, it might be worth to spend in quality, but non-audiophile, speaker cable.

For long distances coax cable is the way to go, as this architecture completely avoids treble loss from cable inductance.
Bigger copper cross-section reduces power loss over long distance.

SOMMER CABLE offers, all reasonably priced for the copper value and make quality involved, and non-esoteric:

2 x 2.5 mm², €6.00 / meter, 4.0 % / -0.35 dB power loss at 10m into 4 Ohm.
2 x 4 mm², €9.14 / meter, 2.6 % / -0.22 dB power loss at 10m into 4 Ohm.
2 x 6 mm², €12.14 / meter, 1.7 % / -0.15 dB power loss at 10m into 4 Ohm.

At 8 Ohm speakers, power losses are halved.
Usually 2 x 2.5 mm² is sufficient.

425-0201F speaker cable SC-Magellan SPK; 2 x 2.50 mm²:
https://shop.sommercable.com/en/Cable/B … 0201F.html

440-0201F speaker cable SC-Magellan SPK; 2 x 4.00 mm²:
https://shop.sommercable.com/en/Cable/B … 0201F.html

460-0201F speaker cable SC-Magellan SPK; 2 x 6.00 mm²:
https://shop.sommercable.com/en/Cable/B … 0201F.html

Matthias may I ask what speakers you drive with the AHB2? I have been eyeing that amp for a while but have Hypex NCores400's and Crimson 630D's on my Amphion 3LS already. Like the latter more than the Hypex. Still would like to try the Benchmark (pun intended).

Self built, sorry.